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/include/init.h b/include/init.h
index 02bb4ce..50a8302 100644
--- a/include/init.h
+++ b/include/init.h
@@ -291,7 +291,7 @@
  *
  * @param total_size	Size of U-Boot (unused?)
  */
-ulong board_get_usable_ram_top(ulong total_size);
+phys_size_t board_get_usable_ram_top(phys_size_t total_size);
 
 int board_early_init_f(void);