dt-bindings: mmc: Document the Amlogic Meson SDHC MMC host controller

dt-bindings: mmc: Document the Amlogic Meson SDHC MMC host controller

This documents the devicetree bindings for the SDHC MMC host controller
found in Meson6, Meson8, Meson8b and Meson8m2 SoCs. It can use a
bus-width of 1/4/8-bit and it supports eMMC spec 4.4x/4.5x including
HS200 mode (up to 100MHz clock). It embeds an internal clock controller
which outputs four clocks (mod_clk, sd_clk, tx_clk and rx_clk) and is
fed by four external input clocks (clkin[0-3]). "pclk" is the module
register clock, it has to be enabled to access the registers.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200512204147.504087-2-martin.blumenstingl@googlemail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>