MAINTAINERS: Add entry for the ASPEED SD/MMC driver

Add myself as the maintainer.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210114031433.2388532-6-andrew@aj.id.au


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>