xilinx: Handle board_get_usable_ram_top(0) properly

xilinx: Handle board_get_usable_ram_top(0) properly

board_get_usable_ram_top() was designed for getting the top most location
for U-Boot allocation that's why function itself supports via total_size
parameter to find out where the right location for U-Boot is.
But function itself is also reused by different (EFI) which is passing
total_size as 0 to find out where the usable ram top is. For this case
doesn't make sense (a waste time) to call any lmb functions.
That's why simply return gd->ram_top.

And gd->ram_top is filled already based on previous call for U-Boot iself.
The same solution is also used by stm32mp by commit 92b611e8b003 ("stm32mp:
correctly handle board_get_usable_ram_top(0)") and commit c8510e397fad
("stm32mp: Fix board_get_usable_ram_top()").

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/44470619e74f3e480b70deac24578e3e0d5c907e.1651225945.git.michal.simek@amd.com