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/board/renesas/ulcb/cpld.c b/board/renesas/ulcb/cpld.c
index ebb2d6f..0c060a5 100644
--- a/board/renesas/ulcb/cpld.c
+++ b/board/renesas/ulcb/cpld.c
@@ -111,7 +111,7 @@
 	if (argc < 3)
 		return CMD_RET_USAGE;
 
-	addr = simple_strtoul(argv[2], NULL, 16);
+	addr = hextoul(argv[2], NULL);
 	if (!(addr == CPLD_ADDR_VERSION || addr == CPLD_ADDR_MODE ||
 	      addr == CPLD_ADDR_MUX || addr == CPLD_ADDR_DIPSW6 ||
 	      addr == CPLD_ADDR_RESET)) {
@@ -122,7 +122,7 @@
 	if (argc == 3 && strcmp(argv[1], "read") == 0) {
 		printf("0x%x\n", cpld_read(dev, addr));
 	} else if (argc == 4 && strcmp(argv[1], "write") == 0) {
-		val = simple_strtoul(argv[3], NULL, 16);
+		val = hextoul(argv[3], NULL);
 		cpld_write(dev, addr, val);
 	}