rockchip: rk3399-gru: Fix max SPL size on bob and kevin

Chromebook bob and kevin typically run coreboot as the initial boot
loader, however, U-Boot proper can be used as a secondary boot loader.
It is also possible to run U-Boot SPL and proper bare metal, with SPL
and the U-Boot payload loaded from SPI flash.

Because of this chromebook bob and kevin only use SPL and not TPL+SPL
like other RK3399 boards, this mean that SPL is loaded to and run from
SRAM instead of DRAM.

The U-Boot payload is located at 0x40000 (256 KiB) offset in SPI flash
and because the BROM only read first 2 KiB for each 4 KiB page, the size
of SPL (idbloader.img) is limited to max 128 KiB.

The chosen bss start address further limits the size of SPL to 120 KiB.

  0xff8e0000 (SPL_BSS_START_ADDR) - 0xff8c2000 (SPL_TEXT_BASE) = 0x1e000

Update SPL_MAX_SIZE to reflect the 120 KiB max size limitation.

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