rockchip: px30: simplify logic for getting SPL boot medium DT node

In preparation of moving spl_perform_fixups to spl-boot-order.c, let's
simplify the logic around mapping the BOOT_DEVICE_x enum index to a DT
node by using an instantiated array of chars instead of creating a new
data structure on the fly.
This will make it easier to factor out the code handling the SPL boot
medium detection by having spl_decode_boot_device common to all SoCs.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>