arm64: xilinx: Move board_get_usable_ram_top() to common location

The commit ce39ee28 ("zynqmp: Do not place u-boot to reserved memory
location") adds functionality for ZynqMP to read reserved memory node and
do not place U-Boot to reserved location. This functionality is generic
across all Xilinx SOCs that's why move it to common location to be used by
all Xilinx SOCs.

On zynq platform this is also fixing issue where U-Boot was placed to
locating which was reserved already which ends up with error message
"ERROR: reserving fdt memory region failed (addr=30000000 size=10000000
flags=4)" which is shown when bdinfo is called.

Tested on vck190, zcu102, zc706 and kc705 to cover all platforms.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/b0817807912f7c7af6a8e1cf9ee04e5ab5de5f6a.1661430188.git.michal.simek@amd.com