mmc: sdhci-spear: add device tree bindings

This adds simple DT bindings for SDHCI SPEAr controller. It uses cd-gpios
from common mmc bindings.

This also fixes spear300-evb.dts with correct name for card detect binding.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>