mmc: android-goldfish: Allow compiling the driver with COMPILE_TEST

mmc: android-goldfish: Allow compiling the driver with COMPILE_TEST

The driver depends on GOLDFISH but there isn't a build dependency
so it's a good idea to allow the driver to always be built when the
COMPILE_TEST option is enabled.

That way, the driver can be built with a config generated by make
allyesconfig and check if a patch would break the build.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>