MAINTAINERS: add RV1108 Rockchip soc to maintained files

The RV1108, as a soc targetted at visual applications, does not follow
the usual RKxxxx naming scheme and thus falls through the cracks of
the current file list for Rockchip socs in MAINTAINERS.

The driver side does not pose a problem, as the rv1108 shares most
functionality with other Rockchip socs as usual, so we only need to
add an expression catching the rv1108* arm32 devicetree files.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Olof Johansson <olof@lixom.net>