MAINTAINERS: Add Krzysztof Kozlowski as maintainer of memory controllers

The generic parts of memory controllers (of_memory.[ch]) lacked any
care.  The memory controller drivers were not abandoned (usually
picked up by architecture maintainers) but in such case I can take care
about them as well.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>