rockchip: rk3568-rock-3a: Update defconfig

Update defconfig for rk3568-rock-3a with new defaults.

Remove the SPL_ROCKCHIP_BACK_TO_BROM=y option, SPL is expected to load
next stage from a FIT image and then jump to next stage not back to
BootRom.

Add CONFIG_SPL_FIT_SIGNATURE=y to let SPL verify an auto generated hash
of FIT images. This help indicate if there is an issue loading any of
the images to DRAM or SRAM.

Extend SPL_MAX_SIZE to 0x40000, SPL is loaded to 0x0 and TF-A is loaded
to 0x40000, use the space in between as SPL_MAX_SIZE.

Add config option to include useful gpio cmd.

Remove the CONFIG_NET_RANDOM_ETHADDR=y option, ethaddr and eth1addr is
set based on cpuid read from OTP.

Filter out assigned-clock props with CONFIG_OF_SPL_REMOVE_PROPS,
U-Boot proper will read and configure assigned-clock props.

Remove the CONFIG_SPL_PMIC_RK8XX=y option, the pmic is not used in SPL.

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