rockchip: rk3399-roc-pc: Fix SPL max size and SPI flash payload offset

TPL max size is limited to 184 KB, SPL is loaded to 0x0 and TF-A is
loaded to 0x40000, this limit SPL max size to 256 KB. With BootRom only
reading first 2 KB per 4 KB page of SPI flash, 880 KB may be needed for
TPL+SPL in a worst-case scenario. (184 KB + 256 KB) x 2 = 880 KB

Use 0xE0000 (896 KB) as the payload offset in SPI flash, this allows
for a payload of 3168 KB before env offset start to overlap.

Also add CONFIG_ROCKCHIP_SPI_IMAGE=y to build a bootable SPI flash
image, u-boot-rockchip-spi.bin.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <foss+u-boot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>