board_f: Fix types for board_get_usable_ram_top()

Commit 37dc958947 ("global_data.h: Change ram_top type to phys_addr_t")
changed type of ram_top member from ulong to phys_addr_t but did not
changed types in board_get_usable_ram_top() function which returns value
for ram_top.

So change ulong to phys_addr_t type also in board_get_usable_ram_top()
signature and implementations.

Fixes: 37dc958947 ("global_data.h: Change ram_top type to phys_addr_t")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

board/xilinx/common/board.c

@@ -631,7 +631,7 @@ int embedded_dtb_select(void)
 #endif
 
 #if defined(CONFIG_LMB)
-ulong board_get_usable_ram_top(ulong total_size)
+phys_size_t board_get_usable_ram_top(phys_size_t total_size)
 {
 	phys_size_t size;
 	phys_addr_t reg;