rockchip: Add SPL_PAD_TO Kconfig default value

rockchip: Add SPL_PAD_TO Kconfig default value

Almost all Rockchip boards use the same Kconfig value for SPL_PAD_TO,
0x7f8000.

u-boot-rockchip.bin is typically written to offset 64S (32KiB) of MMC
media. u-boot.itb (or u-boot.img) is typically expected at offset 16384S
(8MiB) of MMC media (SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x4000).

SPL_PAD_TO is used as the offset for u-boot.itb (or u-boot.img) in the
generated simple-bin binman image, and can be calculated as:

  SPL_PAD_TO = (16384S - 64S) * 512 = 0x7f8000

Add this value as a default value for ARCH_ROCKCHIP.

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