dt-bindings: mmc: Document the Amlogic Meson8 and Meson8b SDIO bindings

This documents the devicetree bindings for the SDIO/MMC host found in
Amlogic Meson8 and Meson8b SoCs. It supports the SD specification v2.0
and the eMMC specification v4.41.
It has an internal "mux" which allows connecting up to three MMC devices
to it. The maximum supported bus-width is 4-bits.

Amlogic's GPL kernel sources call it "SDIO" to differentiate it from the
other MMC controller in (at least the Meson8 and Meson8b) the SoCs (they
call the other one "SDHC", which supports a bus-width of up to 8-bits).

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>