spl: set SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR to 0x4000 for rockchip

Rockchip use a 'loader2' partition for U-Boot, so u-boot.bin or
u-boot.itb load by SPL need to locate at0x4000. Detail here:
http://opensource.rock-chips.com/wiki_Boot_option



Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>