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-keystone/cmd_mon.c b/arch/arm/mach-keystone/cmd_mon.c
index 049d573..e26296b 100644
--- a/arch/arm/mach-keystone/cmd_mon.c
+++ b/arch/arm/mach-keystone/cmd_mon.c
@@ -25,7 +25,7 @@
 
 	freq = CONFIG_SYS_HZ_CLOCK;
 
-	addr = simple_strtoul(argv[1], NULL, 16);
+	addr = hextoul(argv[1], NULL);
 
 	header = (struct image_header *)addr;
 
@@ -40,7 +40,7 @@
 	       size);
 
 	if (argc >=  3)
-		ecrypt_bm_addr = simple_strtoul(argv[2], NULL, 16);
+		ecrypt_bm_addr = hextoul(argv[2], NULL);
 
 	rcode = mon_install(load_addr, dpsc_base, freq, ecrypt_bm_addr);
 	printf("## installed monitor @ 0x%x, freq [%d], status %d\n",
@@ -76,8 +76,8 @@
 	if (argc < 3)
 		return CMD_RET_USAGE;
 
-	core_id = simple_strtoul(argv[1], NULL, 16);
-	on = simple_strtoul(argv[2], NULL, 16);
+	core_id = hextoul(argv[1], NULL);
+	on = hextoul(argv[2], NULL);
 
 	if (on)
 		rcode = mon_power_on(core_id, fn);