MAINTAINERS: mmc: Cleanup MMC/SD/SDIO section and SDHCI driver section

As Chris Ball has moved on to other assignments, he's no longer able to
help me maintain MMC. Let's remove him from the MMC sections in
MAINTAINERS and add him to CREDIT file.

This also affects the SDHCI DRIVER section, since its state now becomes
orphan.

Cc: Chris Ball <chris@printf.net>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Chris Ball <chris@printf.net>