trusty: Add simple fuzzer for keymaster TA

Adds a simple libfuzzer-based coverage guided fuzzer for keymaster.
Current coverage is low, so we'll need to improve this with an initial
corpus or a more sophisticated protocol aware fuzzer.

Bug: 175918322
Test: make trusty_keymaster_fuzzer
Test: adb shell /data/fuzz/arm64/trusty_keymaster_fuzzer/trusty_keymaster_fuzzer
Change-Id: Iad1713e1a73cc5a6a3ec742cc433d1337aca9bc3