MAINTAINERS: Add maintainer entry for the imx8 DDR PMU driver

Frank Li wrote the imx8 DDR PMU driver and has access to the hardware,
so add him as maintainer for the files in question.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[will: fixed case of title]
Signed-off-by: Will Deacon <will.deacon@arm.com>