rockchip: rk3399-puma: remove unnecessary simple-bin:fit:offset override

Since commit 6007b69d ("rockchip: rk3399-puma: Update SPL_PAD_TO
Kconfig option"), SPL_PAD_TO matches
(CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR - 64) * 512 and the default
value for simple-bin:fit:offset in rockchip-u-boot.dtsi is
SPL_PAD_TO, so let's remove this override.

Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>