MAINTAINERS: Add Jonathan Cameron to drivers/cache and add lib/cache_maint.c + header

MAINTAINERS: Add Jonathan Cameron to drivers/cache and add lib/cache_maint.c + header

Seems unfair to inflict the cache-coherency drivers on Conor with out also
stepping up as a second maintainer for drivers/cache.

Include the library support for cache-coherency maintenance drivers to the
existing entry.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>