rockchip: rock-pi-4: Use SDMA to boost eMMC performance

rockchip: rock-pi-4: Use SDMA to boost eMMC performance

Enable the use of SDMA mode to boost eMMC performance on ROCK Pi 4.
Also add missing flags to indicate the supported MMC modes.

Using mmc read command to read 32 MiB data shows following improvement:

  => time mmc read 10000000 2000 10000

Before: time: 3.178 seconds
After: time: 0.402 seconds

This also enables CONFIG_SPL_FIT_SIGNATURE option to help discover
any possible future issue with loading TF-A into DRAM/SRAM.

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