board_f: Fix types for board_get_usable_ram_top()

Commit 37dc958947ed ("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: 37dc958947ed ("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>
diff --git a/board/ti/am65x/evm.c b/board/ti/am65x/evm.c
index 8a0a506..34ec391 100644
--- a/board/ti/am65x/evm.c
+++ b/board/ti/am65x/evm.c
@@ -61,7 +61,7 @@
 	return 0;
 }
 
-ulong board_get_usable_ram_top(ulong total_size)
+phys_size_t board_get_usable_ram_top(phys_size_t total_size)
 {
 #ifdef CONFIG_PHYS_64BIT
 	/* Limit RAM used by U-Boot to the DDR low region */