global: Convert simple_strtoul() with decimal to dectoul()

It is a pain to have to specify the value 10 in each call. Add a new
dectoul() function and update the code to use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/compulab/common/omap3_display.c b/board/compulab/common/omap3_display.c
index cb9ebae..4ed3b9c 100644
--- a/board/compulab/common/omap3_display.c
+++ b/board/compulab/common/omap3_display.c
@@ -244,7 +244,7 @@
 	int divisor, pixclock_val;
 	char *pixclk_start = pixclock;
 
-	pixclock_val = simple_strtoul(pixclock, &pixclock, 10);
+	pixclock_val = dectoul(pixclock, &pixclock);
 	divisor = DIV_ROUND_UP(PIXEL_CLK_NUMERATOR, pixclock_val);
 	/* 0 and 1 are illegal values for PCD */
 	if (divisor <= 1)