rockchip: adding the missing "/" in entries of boot_devices

Without the prefix, "same-as-spl" in `u-boot,spl-boot-order` will not work
as expected. When board_boot_order() `spl-boot-order.c` meets
"same-as-spl", it gets the conf by looking the boot_devices table by boot
source, and parse the node by the conf with:

   node = fdt_path_offset(blob, conf);

which will failed without the "/" indicating the path.

Currently only entries of boot_devices in rk3399 have the "/" prefix.
Therefore add the missing ones in other boards.

Signed-off-by: Levin Du <djw@t-chip.com.cn>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>