mmc: core: add DT bindings for eMMC HS400 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.
Specially, if host can support HS400, it means that host can also
support HS200.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>