Peach Pit and Pi boards have a WiFi module that is always powered
but needs toggling an enable pin and ungating a 32kHz reference
clock as part of their power sequencing.

Add a dev node for the SDIO slot and a MMC power sequence provider.
Signed-off-by: NJavier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: NKukjin Kim <kgene@kernel.org>