mmc: core: Add DT bindings for eMMC high-speed DDR 1.8/1.2V

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>