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 <[e-mail]>
Acked-by: Lee Jones <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

MAINTAINERS

···
 F:	drivers/phy/phy-stih41x-usb.c
 F:	drivers/pinctrl/pinctrl-st.c
 F:	drivers/remoteproc/st_remoteproc.c
+F:	drivers/remoteproc/st_slim_rproc.c
 F:	drivers/reset/sti/
 F:	drivers/rtc/rtc-st-lpc.c
 F:	drivers/tty/serial/st-asc.c
···
 F:	drivers/usb/host/ohci-st.c
 F:	drivers/watchdog/st_lpc_wdt.c
 F:	drivers/ata/ahci_st.c
+F:	include/linux/remoteproc/st_slim_rproc.h
 
 ARM/STM32 ARCHITECTURE
 M:	Maxime Coquelin <[e-mail]>