rockchip: rk3399-gru: Enable dcache and signature validation in SPL

rockchip: rk3399-gru: Enable dcache and signature validation in SPL

With TPL being used to init DRAM, SPL being used to load FIT and the
adjusted FIT payload offset it is now possible to increase the size
limit of SPL to 256 KB and enable uses of dcache and FIT signature
validation.

Drop SPL_SYS_DCACHE_OFF=y to enable use of dcache in SPL.

Drop SPL_FIT_SIGNATURE=n to enable signature validation of FIT in SPL.

Change SPL_MAX_SIZE to 256 KB now that payload offset has moved in SPI
and TF-A may be loaded to 0x40000 in DRAM.

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