mmc: SD/MMC Host Controller for Wondermedia WM8505/WM8650

This patch adds support for the SD/MMC host controller found
on Wondermedia 8xxx series SoCs, currently supported under
arm/arch-vt8500.

A binding document is also included, based on mmc.txt with
additional properties.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Chris Ball <cjb@laptop.org>