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/powerpc/cpu/mpc85xx/mp.c b/arch/powerpc/cpu/mpc85xx/mp.c
index 653efe0..b1b002c 100644
--- a/arch/powerpc/cpu/mpc85xx/mp.c
+++ b/arch/powerpc/cpu/mpc85xx/mp.c
@@ -167,7 +167,7 @@
 	for (i = 1; i < 3; i++) {
 		if (argv[i][0] != '-') {
 			u8 entry = boot_entry_map[i];
-			val = simple_strtoul(argv[i], NULL, 16);
+			val = hextoul(argv[i], NULL);
 			table[entry] = val;
 		}
 	}