Introduce CONFIG_GOOGLE_FIRMWARE

In order to keep Google's firmware drivers organized amongst themselves,
all Google firmware drivers are gated on CONFIG_GOOGLE_FIRMWARE=y, which
defaults to 'n' in the kernel build.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>