mmc: Kconfig: remove i.MX individual SoC dependency

The individual SoC dependency in Kconfig hardly scales anymore.
Instead of having such a fine grained dependency just depend
on ARCH_MXC and risk that the uninformed user has to look in
the help text to figure out which driver is the correct one.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>