rockchip: rk3399: update MAINTAINER file

This patch add maintainer information for rk3399 evb.
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>

board/rockchip/evb_rk3399/MAINTAINERS

+EVB-RK3399
+M:      Kever Yang <kever.yang@rock-chips.com>
+S:      Maintained
+F:      board/rockchip/evb_rk3399
+F:      include/configs/evb_rk3399.h
+F:      configs/evb-rk3399_defconfig