clk: hisilicon: add hi3620_mmc_clks

Suggest by Arnd: abstract mmc tuning as clock behavior,
also because different soc have different tuning method and registers.
hi3620_mmc_clks is added to handle mmc clock specifically on hi3620.

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>