mmc: sh_mobile_sdhi: add support for 2 clocks

Some controllers have 2 clock sources instead of 1. The 2nd clock
is for the internal card detect logic and must be enabled/disabled
along with the main core clock for proper operation.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>