sh: sh7722 Enable SDIO IRQs

This patch enables the interrupt generation for SDIO IRQs
of the sdhi controllers of the SoC. To make sure SDIO IRQs
are used, announce the MMC_CAP_SDIO_IRQ capability
on migor.

Signed-off-by: Arnd Hannemann <arnd@arndnet.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>