mmc: dw_mmc: use the mmc_of_parse() instead of local parser

mmc_of_parse() have been already parsed the general capability.
Didn't need to use the local parser.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>