rockchip: puma-rk3399: remove useless CONFIG_SYS_SPI_U_BOOT_OFFS

The SPL payload offset when booting from SPI defaults to
CONFIG_SYS_SPI_U_BOOT_OFFS but can be overridden by
u-boot,spl-payload-offset. The Device Tree for Puma Haikou has this
property so there's no need to have this one option in the defconfig,
especially since they are not in sync and therefore confusing.

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>