rockchip: rk3328: Add support to build bootable SPI image

Similar to RK35xx the BootRom in RK3328 can read all data and look for
idbloader at 0x8000, same as it does for SD and eMMC.

Use the rksd format and modify the mkimage offset to generate a bootable
u-boot-rockchip-spi.bin that can be written to 0x0 of SPI NOR flash.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>