rockchip: dts: rk3328: rock64: Add same-as-spl order

rk3328 can use same-as-spl option so next loaders are loaded from the same
medium.
Add the boot order in the rock64 dts otherwise booting from sdcard
will result in u-boot looking into the eMMC.

Signed-off-by: Emmanuel Vadot <manu@freebsd.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>