MAINTAINERS: mmc: Add Adrian Hunter as the maintainer for SDHCI

up the maintainer role for SDHCI. I am very pleased that Adrian Hunter
volunteered and accepted the challenge!

The SDHCI code is currently in quite poor quality, but we have agreed on
a way forward to try to reach a point where the SDHCI core becomes more of
a set of library functions. Each SDHCI driver variant can then decide
which functions to use and allows it to implement variant specific code,
without needing to also change SDHCI core code.

In the end we aim to get better optimized and maintainable code.

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