global: Convert simple_strtoul() with hex to hextoul()

It is a pain to have to specify the value 16 in each call. Add a new
hextoul() function and update the code to use it.

Add a proper comment to simple_strtoul() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-stm32mp/cmd_stm32key.c b/arch/arm/mach-stm32mp/cmd_stm32key.c
index 50840b0..68f2892 100644
--- a/arch/arm/mach-stm32mp/cmd_stm32key.c
+++ b/arch/arm/mach-stm32mp/cmd_stm32key.c
@@ -162,7 +162,7 @@
 		return CMD_RET_SUCCESS;
 	}
 
-	addr = simple_strtoul(argv[1], NULL, 16);
+	addr = hextoul(argv[1], NULL);
 	if (!addr)
 		return CMD_RET_USAGE;
 
@@ -185,7 +185,7 @@
 		yes = true;
 	}
 
-	addr = simple_strtoul(argv[argc - 1], NULL, 16);
+	addr = hextoul(argv[argc - 1], NULL);
 	if (!addr)
 		return CMD_RET_USAGE;