MAINTAINERS: Add st slim core rproc driver to STi section.

This patch adds the slim core rproc driver to the STi section
of the MAINTAINERS file.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>