mmc: sh_mmcif: add regulator support

Add regulator support to the sh_mmcif driver, but also preserve the current
power-callback.

Also note, that the card power is not switched off during clock gating
periods, hence there's no need to power it on every time the card is
re-activated.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviwed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>