To support SOCs which specifies specific MMC power sequences, document
some MMC DT bindings to be able to describe these hardwares.

Let's also document bindings for a simple MMC power sequence provider,
which purpose is to support a set of common properties between various
SOCs.

In this initial step, let's also document a top level description of
the MMC power sequence and describe the compatible string used for the
simple MMC power sequence provider.

The simple MMC power sequence provider will initially support a reset
GPIO. From several earlier posted patches, it's clear that such
hardware exists. Especially some WLAN chips which are attached to an
SDIO interface may use a GPIO reset.
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>
Tested-by: NJavier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: NJavier Martinez Canillas <javier.martinez@collabora.co.uk>