rockchip: rk3588-edgeble-neu6: Enable FIT checksum validation

rockchip: rk3588-edgeble-neu6: Enable FIT checksum validation

Enable SPL_FIT_SIGNATURE=y and LEGACY_IMAGE_FORMAT=y to validate FIT
images checksum in SPL.

Change to SPL_MAX_SIZE=0x40000 to allow SPL up to 256 KiB in size.

Enable SPL_DM_SEQ_ALIAS=y and SPL_PINCTRL=y to ensure eMMC and SD-card
pinctrl is configured in SPL.

Enable SYS_NS16550_MEM32=y to use readl/writel in serial driver.

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