mfd: cros_ec: add Kunit tests

./tools/testing/kunit/kunit.py run \
	--arch=x86_64 \
	--kconfig_add CONFIG_CHROME_PLATFORMS=y \
	--kconfig_add CONFIG_CROS_EC=y \
	--kconfig_add CONFIG_DEBUG_KERNEL=y \
	--kconfig_add CONFIG_DEBUG_INFO_DWARF5=y \
	--raw_output=all \
	--kernel_args "no_hash_pointers" \
	cros_ec_dev*
	#--kconfig_add CONFIG_MFD_CROS_EC_DEV=y \

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>