Travis: reduce ARM itest parallelism to 2

Reduces the number of concurrent tranches of itests running to two on
ARM in an attempt to make the tests less flaky because of very high CPU
usage with the default 4 parallel tranches.

docs/release-notes/release-notes-0.14.0.md

@@ -150,6 +150,9 @@ you.
   lead to failed tests sometimes when the CPU of the GitHub CI runner was
   strained too much.
 
+* [Reduce the number of parallel itest runs to 2 on
+  ARM](https://github.com/lightningnetwork/lnd/pull/5731).
+
 ## Documentation
 
 * [Outdated warning about unsupported pruning was replaced with clarification that LND **does**