rpmsg: add Kconfig menu

Add a dedicated Kconfig menu for the rpmsg drivers, so they
don't show up in the main driver menu.
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>

drivers/rpmsg/Kconfig

+menu "Rpmsg drivers"
+
 # RPMSG always gets selected by whoever wants it
 config RPMSG
 	tristate
 	select VIRTIO
 	select VIRTIO_RING
+
+endmenu