Add USDHC driver to support boot EMMC. Only initialization
and single/multiple block read are tested.

[bod: fixed checkpatch.pl complaints]
[bod: changed name to imx_usdhc for namespace consistency]
[bod: squashed antecedent fixes into this one patch]
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>