platform/chrome: cros_ec_proto: Allow to build as module

platform/chrome: cros_ec_proto: Allow to build as module

Allow to build ChromeOS EC communication protocol helpers as
kernel module which is particularly useful for Android GKI (Generic
Kernel Image) configuration. With this change the ChromeOS Platform
(CONFIG_CHROME_PLATFORMS=y) can be enabled directly from the vendor
kconfig fragment whithout additional configuration in gki_defconfig.

Signed-off-by: Slawomir Rosek <srosek@chromium.org>
Link: https://lore.kernel.org/r/20250328132612.511471-1-srosek@chromium.org
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>