mmc: mvsdio: implement a Device Tree binding

This patch adds a simple Device Tree binding for the mvsdio driver, as
well as the necessary documentation for it. Compatibility with non-DT
platforms is preserved, by keeping the platform_data based
initialization.

We introduce a small difference between non-DT and DT platforms: DT
platforms are required to provide a clocks = <...> property, which the
driver uses to get the frequency of the clock that goes to the SDIO
IP. The behaviour on non-DT platforms is kept unchanged: a clock
reference is not mandatory, but the clock frequency must be passed in
the "clock" field of the mvsdio_platform_data structure.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Stefan Peter <s.peter@mpl.ch>
Tested-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Chris Ball <cjb@laptop.org>