rockchip: veyron: Add a note about the SDRAM voltage

Add a comment to indicate that we are not supporting the PWM regulator
yet.
Signed-off-by: NSimon Glass <sjg@chromium.org>

board/google/veyron/veyron.c

@@ -5,3 +5,9 @@
  */
 
 #include <common.h>
+
+/*
+ * We should increase the DDR voltage to 1.2V using the PWM regulator.
+ * There is a U-Boot driver for this but it may need to add support for the
+ * 'voltage-table' property.
+ */