mmc: add DT bindings for more MMC capability flags

Many MMC capability flags are platform-dependent and are traditionally set
in platform data. With DT often each such capability requires a special
binding. Add bindings for MMC_CAP_SD_HIGHSPEED, MMC_CAP_MMC_HIGHSPEED,
MMC_CAP_POWER_OFF_CARD and MMC_CAP_SDIO_IRQ capabilities. Also add code to
DT parser to look up "keep-power-in-suspend" and "enable-sdio-wakeup"
bindings and set MMC_PM_KEEP_POWER and MMC_PM_WAKE_SDIO_IRQ respectively,
if found.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>