ARM: mach-shmobile: add missing GPIO IRQ configuration on mackerel

SDHI0 card-detect GPIO IRQ on mackarel currently works, because it is the
default configuration of IRQ26. However, we should not rely on this and
should configure the function explicitly.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>