mmc: core: Add DT bindings for eMMC hardware reset support

Sometime only need set MMC_CAP_HW_RESET for one of MMC hosts,
So set it in device tree is better.

Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>