soc: imx: add Kconfig symbols for blk-ctrl drivers

Currently the dependencies of the blk-ctrl drivers are not fully
described in Kconfig, which can trip over the compile tests on
platforms where those drivers are not usually enabled. Add a
non user-selectable symbol to be describe those dependencies.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>