MAINTAINERS: add cache binding directory to cache driver entry

The directory covering cache controller bindings has no MAINTAINER other
than the fallback to myself, Rob and Krzysztof. Add it to the entry for
the corresponding drivers.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>