mmc: sdhci-iproc: support standard byte register accesses

Add bytewise register accesses support for newer versions of IPROC
SDHCI controllers.
Previous sdhci-iproc versions of SDIO controllers
(such as Raspberry Pi and Cygnus) only allowed for 32-bit register
accesses.

Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>