rockchip: puma-rk3399: migrate to u-boot-rockchip.bin

The offset of the SPL payload on Puma is different than for other
Rockchip devices in that it is stored at offset 256K instead of much
further away in the MMC.

Flashing one binary instead of two at different offsets is much more
user friendly so let's migrate to it by modifying the offset in the Puma
specific Device Tree.

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