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>
diff --git a/doc/board/rockchip/rockchip.rst b/doc/board/rockchip/rockchip.rst
index ac4dcce..c1de34b 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -173,7 +173,8 @@
 
 .. code-block:: bash
 
-        export BL31=../rkbin/bin/rk35/rk3568_bl31_v1.34.elf
+        export BL31=../arm-trusted-firmware/build/rk3568/release/bl31/bl31.elf
+        [or]export BL31=../rkbin/bin/rk35/rk3568_bl31_v1.34.elf
         export ROCKCHIP_TPL=../rkbin/bin/rk35/rk3568_ddr_1560MHz_v1.13.bin
         make evb-rk3568_defconfig
         make CROSS_COMPILE=aarch64-linux-gnu-