MAINTAINERS: memory: cover also header file

Add include/memory/ in memory drivers maintainers entry.  So far there
is only one file there for Renesas Reduced Pin Count Interface driver.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210601160608.312446-2-krzysztof.kozlowski@canonical.com