mmc: dw_mmc: make IO accessors endian agnostic

The dw_mmc driver does not use endian agnostic IO accessors, so fix
the use of __raw reads and writes to be the relaxed versions.

This fixes the dw_mmc driver initialisation on Altera socfpga in big endian.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>