ARM: shmobile: mackerel: switch SDHI and MMCIF interfaces to slot-gpio

Both SDHI and MMCIF drivers can use the standard slot-gpio card-detection
functions. Switch mackerel to using them instead of platform callbacks.

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>