ARM: mach-shmobile: add support for the second SD-/MMC-card slot on ap4evb

The ap4evb board is equipped with two card slots: one SD-card slot and one
SD-/MMC-card slot. The latter is connected to the second SDHI interface on
sh7372 (SDHI1). Its power supply can be jumpered either to 1.8 or 3.3V, we fix
it at default 1.8V for now.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>