firmware: remove unnecessary patterns from .gitignore

Commit df85b2d7 ("firmware: Restore support for built-in firmware")
should not have restored this .gitignore blindly.

*.gen.S is only pattern that should be ignored by git.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

firmware/.gitignore

 *.gen.S
-*.fw
-*.bin
-*.csp
-*.dsp
-ihex2fw