mmc: mmci: Add register read/write wrappers.

This patch adds wrappers for readl/writel functions used in the driver. The
reason for this wrappers is to accommodate SOCs like Qualcomm which has
requirement for delaying the write for few cycles when writing to its SD Card
Controller registers.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>