[amlogic-video] .gitignore the cipd firmware .bin

This ignores the amlogic-video firmware directory so that git status
won't be noisy.  This change assumes that we'll keep cipd content in a
firmware/ directory to itself (ignores the dir, not the two files
separately).

Tested: ran "git status" after adding the line to .gitignore
Change-Id: Id970838cc2e1c2233b8dccd5c8fe5597dc893c3a

.gitignore

@@ -30,3 +30,4 @@ tools/cipd.gni
 *.iml
 **/Cargo.lock
 **/Cargo.toml
+drivers/video/amlogic-decoder/firmware/