mmc: pwrseq_simple: add to_pwrseq_simple() macro

This patch adds to_pwrseq_simple() macro to make the code more readable.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>