imx233: make button-lradc drive able to handle VDDIO derived values

In most devices, the button ladder is not actually derived from VDDIO but
from a constant voltage source, making it very easy to read it. However on
some devices like ther ZEN X-Fi Style, the ladder is wired to VDDIO we
can be changed so it's crucial that the button driver correctly scales the
values wrt VDDIO.

Change-Id: Ifc11abe2838fa7d16d0d60ecd96964a8dc5ea6d7

firmware/target/arm/imx233/lradc-imx233.h

@@ -65,6 +65,9 @@
 /* frequency of the delay counter */
 #define LRADC_DELAY_FREQ    2000
 
+/* maximum value of a sample (without accumulation), defines the precision */
+#define LRADC_MAX_VALUE     4096
+
 typedef void (*lradc_irq_fn_t)(int chan);
 
 void imx233_lradc_init(void);