rockchip: rk35xx: Remove use of eMMC DDR52 mode

Testing has shown that writing to eMMC using DDR52 mode does not seem to
work on RK356x and RK3588 boards.

A simple test of writing a single block to e.g. sector 0x4000 fails:

  # Rescan using DDR52 mode
  => mmc rescan 4

  # Write a single block to sector 0x4000 fails with ERROR
  => mmc write 20000000 4000 1

With the MMC_SPEED_MODE_SET Kconfig option enabled.

Fix this by removing the mmc-ddr-1_8v prop from sdhci nodes in affected
board u-boot.dtsi files.

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