Documentation: synopsys-dw-mshc: add binding for reset-names

Add reset-names property for binding dw-mmc controller.
It might be used together with "reset" property.
-  Note: It must be "reset" as name.

Fixes: d6786fef ("mmc: dw_mmc: add reset support to dwmmc host controller")
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>