[media] ov2640: add information about DSP register 0xc7

According to ov2640 software application notes, there are two Automatic
White Balance (AWB) modes, which are selected by DSP register 0xc7:

1) Simple AWB: assumes the average color is gray
   + independent from lens
   - doesn't work well if captured area contains unbalanced colors
     (e.g. large blue background)

2) Advanced AWB: uses color temperature information
   + more accurate, works with all image contents
   - lens specific, requires calibration
Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/i2c/ov2640.c

@@ -106,6 +106,10 @@
 #define   CTRL1_AWB_GAIN     0x04
 #define   CTRL1_LENC         0x02
 #define   CTRL1_PRE          0x01
+/*      REG 0xC7 (unknown name): affects Auto White Balance (AWB)
+ *	  AWB_OFF            0x40
+ *	  AWB_SIMPLE         0x10
+ *	  AWB_ON             0x00	(Advanced AWB ?) */
 #define R_DVP_SP    0xD3 /* DVP output speed control */
 #define   R_DVP_SP_AUTO_MODE 0x80
 #define   R_DVP_SP_DVP_MASK  0x3F /* DVP PCLK = sysclk (48)/[6:0] (YUV0);
@@ -449,7 +453,7 @@ static const struct regval_list ov2640_init_regs[] = {
 	{ 0xc5,   0x11 },
 	{ 0xc6,   0x51 },
 	{ 0xbf,   0x80 },
-	{ 0xc7,   0x10 },
+	{ 0xc7,   0x10 },	/* simple AWB */
 	{ 0xb6,   0x66 },
 	{ 0xb8,   0xA5 },
 	{ 0xb7,   0x64 },