mmc: sdhci-brcmstb: Add driver for Broadcom BRCMSTB SoCs

Add SDHCI driver for Broadcom BRCMSTB SoCs.
This driver works with all ARM based SoCs and the 7425, 7429
and 7435 MIPS based SoCs.

The driver disables all UHS speed modes by default and relies
on the Device Tree node properties to enable these modes for
SoC/Board combinations that support them.

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>