mmc: sdhci: Use 'void *' for not 'u8 *' for ADMA data

It is kernel-style to use 'void *' for anonymous data.
This is being applied to the ADMA bounce buffer which
contains unaligned bytes, and to the ADMA descriptor
table which will contain 32-bit ADMA descriptors
or 64-bit ADMA descriptors when support is added.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>