ARM: arch-shmobile: support SDHI card detection on mackerel, using a GPIO

On sh7372 the card-detection pin of SDHI0 can also produce interrupts,
when configured as GPIO. Use this feature to power down SDHI0, when no
card is plugged in.

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