rockchip: px30-ringneck: Update SPL_PAD_TO Kconfig option

On px30-ringneck the FIT payload is located at sector 0x200 compared to
the more Rockchip common sector 0x4000 offset:
	SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x200

Because FIT payload is located at sector 0x200 and the TPL+SPL is
located at sector 64, the combined size of TPL+SPL cannot take up more
than 224KiB:
	(0x200 - 64) x 512 = 0x38000 (224 KiB)

Adjust SPL_PAD_TO to match the used 0x200 sector offset.

While at it, update the px30-ringneck-u-boot.dtsi to remove the now
unnecessary override of simple-bin:fit:offset since SPL_PAD_TO matches
with the current formula.

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