ARM: u300: Convert to the common mmc DT bindings for highspeed mode

The mmci host driver supports the common mmc DT parser, which enables
us to use the use common names instead.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>