mfd: Add MFD_CROS_EC dependencies

The MFD_CROS_EC symbol select CHROME_PLATFORMS and CROS_EC_PROTO but
that caused a Kconfig unmet direct dependencies warning since these
symbols could only be selected for X86 || ARM.

The fix it, the CHROME_PLATFORMS dependencies were relexed on commit
d12bbcd3 ("platform/chrome: Don't make CHROME_PLATFORMS depends
on X86 || ARM") but that was found to be wrong and the correct fix
is to add the needed dependencies to the MFD_CROS_EC config symbol.

There are only x86 and ARM based Chromebooks so to avoid showing up
the config option on unsupported platforms, make the symbol depend on
these architectures. Also add a || COMPILE_TEST so it can have build
coverage on other platforms.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>