mmc: sdhi: Enable driver compilation with COMPILE_TEST

This helps increasing build testing coverage.

The driver doesn't compile on (at least) x86 due (possibly among others)
to missing readsw/writesw I/O accessors, restrict compilation to SUPERH
or ARM.

Whether the CTL_DMA_ENABLE register is part of the standard TMIO
controller or is Renesas-specific is unknown and impossible to test as
we have no current or planned TMIO DMA users other than SUPERH and
ARCH_SHMOBILE. Writing to the register is thus conditionally compiled
for SUPERH and ARCH_SHMOBILE only. Adding ARCH_SHMOBILE_MULTI to the
list would extend this to multiarch kernels, but would break the driver
for non-shmobile platforms if the register is Renesas-specific. We can
thus get rid of the conditional compilation completely without
introducing any further issue, and let future non-Renesas users deal
with the situation if it turns out to be a the problem.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>