mmc: sdhci-pltfm: Add a common clk API implementation of get_timeout_clock

Quite a few drivers have a implementation of the get_timeout_clock
callback which simply returns the result of clk_get_rate on the device's
clock. This patch adds a common implementation of this to the sdhci-pltfm
module and replaces all custom implementations with the common one.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Kevin Liu <kliu5@marvell.com>
Signed-off-by: Chris Ball <cjb@laptop.org>