mmc: sh_mmcif: calculate best clock with parent clock

MMCIF IP on R-Car series has parent clock which can be set several rate,
and it was not implemented on old SH-Mobile series (= SH-Mobile series
parent clock was fixed rate) R-Car series MMCIF can use more high speed
access if it setups parent clock. This patch adds parent clock setup
method. It will be used if DT has "max-frequency", and then, this driver
assumes it is booted on R-Car Gen2 or later SoC. Because SH-Mobile series
(which doesn't boot from DT) and R-Car series (which boots from DT) have
different divider.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
[Ulf: Silence compiler warning]