ARM: shmobile: mackerel: clean up MMCIF vs. SDHI1 selection

MMCIF and SDHI1 share the same slot on mackerel. One of them is selected
by a jumper, which cannot be queried from software. Currently in software
one of the two interfaces is selected, depending whether or not the MMCIF
driver is enabled. This is not optimal, since the kernel has to be rebuilt
to switch from one interface to another. Still, so far there isn't a
better option. At least make this selection consistent.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>