arm64: zynqmp: Remove SOM *u-boot.dtsi

Disable mmc from u-boot.dtsi file because it was there only for kv260
board. With kr260 this is not needed because we will switch to full DT per
board with SD/EMMC there too.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/3440d9f94361b4800658f313a5785f43ee84ecf3.1642590109.git.michal.simek@xilinx.com