evb-rk3568: Update MAINTAINERS and documentation

Update the MAINTAINERS file to include the devicetree for the
rk3568-evb1-v10 board.

Also update Rockchip board docs to include information on building
RK3568 based devices.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>