mmc: core: Add DT bindings for SD card's UHS bus speed modes

Provide the option to configure these speed modes per host, for those
host driver's that can't distinguish this in runtime.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>