IOS-XR get_lldp_neighbor_details does not support 'N/A' capabilities
### Description of Issue/Question

The constants LLDP_CAPAB_TRANFORM_TABLE does not include 'N/A' of system capabilities. No checks for this value either.
```
<SystemCapabilities>
     N/A
</SystemCapabilities>
```


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### Did you follow the steps from https://github.com/napalm-automation/napalm#faq
(Place an ``x`` between the square brackets where applicable)

- [x] Yes
- [] No


### Setup

### napalm version
(Paste verbatim output from `pip freeze | grep napalm` between quotes below)

```
root@salt-master1:/# python3 -m pip freeze | grep napalm
napalm==3.2.0
```

### Network operating system version
(Paste verbatim output from `show version` - or equivalent - between quotes below)

```
RP/0/RP0/CPU0:hbr1.12345.999#show version
Wed Sep 23 18:41:02.500 CEST
Cisco IOS XR Software, Version 7.1.2 LNT
Copyright (c) 2013-2020 by Cisco Systems, Inc.

Build Information:
 Built By     : ahoang
 Built On     : Sat Aug 29 20:04:27 UTC 2020
 Build Host   : iox-lnx-069
 Workspace    : /auto/srcarchive13/prod/7.1.2/ncs540l/ws
 Version      : 7.1.2
 Label        : 7.1.2

cisco NCS540L (C3708 @ 1.70GHz)
System uptime is 1 week, 1 day, 10 hours, 2 minutes

```

### Steps to Reproduce the Issue

### Error Traceback
(Paste the complete traceback of the exception between quotes below)

```
================= Traceback =================

Traceback (most recent call last):
  File "/usr/local/bin/napalm", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.8/dist-packages/napalm/base/clitools/cl_napalm.py", line 308, in main
    run_tests(args)
  File "/usr/local/lib/python3.8/dist-packages/napalm/base/clitools/cl_napalm.py", line 291, in run_tests
    call_getter(device, args.method, **method_kwargs)
  File "/usr/local/lib/python3.8/dist-packages/napalm/base/clitools/cl_napalm.py", line 27, in wrapper
    r = func(*args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/napalm/base/clitools/cl_napalm.py", line 255, in call_getter
    r = func(**kwargs)
  File "/usr/local/lib/python3.8/dist-packages/napalm/iosxr/iosxr.py", line 852, in get_lldp_neighbors_detail
    "remote_system_capab": napalm.base.helpers.transform_lldp_capab(
  File "/usr/local/lib/python3.8/dist-packages/napalm/base/helpers.py", line 463, in transform_lldp_capab
    [constants.LLDP_CAPAB_TRANFORM_TABLE[c.strip()] for c in capabilities]
  File "/usr/local/lib/python3.8/dist-packages/napalm/base/helpers.py", line 463, in <listcomp>
    [constants.LLDP_CAPAB_TRANFORM_TABLE[c.strip()] for c in capabilities]
KeyError: 'n/a'
```

diff --git a/napalm/base/constants.py b/napalm/base/constants.py
index 75f492b1..d505081a 100644
--- a/napalm/base/constants.py
+++ b/napalm/base/constants.py
@@ -69,6 +69,7 @@ NETMIKO_MAP = {
 }
 LLDP_CAPAB_TRANFORM_TABLE = {
     "o": "other",
+    "n/a": "other",
     "p": "repeater",
     "b": "bridge",
     "w": "wlan-access-point",
['napalm/base/constants.py']