arm64: zynqmp: Add note about UHS mode on some boards

Add note about UHS mode and add no-1-8-v property to zc1751-dc1 board.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/462b95844e7aedb00768035913265d7af90c3b2f.1623684253.git.michal.simek@xilinx.com