mmc: rockchip_sdhci: Limit number of blocks read in a single command

Using DMA to load TF-A into SRAM fails when booting from eMMC on RK3588.

  ## Checking hash(es) for Image atf-3 ... sha256 error!
  Bad hash value for 'hash' hash node in 'atf-3' image node
  spl_load_simple_fit: can't load image loadables index 2 (ret = -1)
  mmc_load_image_raw_sector: mmc block read error

Fix this by using PIO mode in SPL and limit the number of blocks used in
a single read command to avoid triggering Data End Bit Error interrupt.

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