MAINTAINERS: rockchip: add board/rockchip as maintained entry

Directory board/rockchip/ are all boards for Rockchip SoCs,
so add it to maintained entry.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

MAINTAINERS

@@ -135,6 +135,7 @@ M:	Simon Glass <sjg@chromium.org>
 S:	Maintained
 T:	git git://git.denx.de/u-boot-rockchip.git
 F:	arch/arm/mach-rockchip/
+F:	board/rockchip/
 
 ARM SAMSUNG
 M:	Minkyu Kang <mk7.kang@samsung.com>