cmd: add new command to read edid

Add a new command to read EDID info from connected display.

When applicable EDID can also be retrieved by commands such as:

  i2c dev x
  i2c edid 0x50

but the new read_edid function relies on the implementation of the
read_edid callback from DISPLAY driver.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>

include/display.h

@@ -25,6 +25,16 @@ struct display_plat {
 	bool in_use;
 };
 
+/**
+ * display_read_edid() - Read edid from display
+ *
+ * @dev:	Device to read from
+ * @buf:	Buffer to read into (should be EDID_SIZE bytes)
+ * @buf_size:	Buffer size (should be EDID_SIZE)
+ * Return number of bytes read, <= 0 for error
+ */
+int display_read_edid(struct udevice *dev, u8 *buf, int buf_size);
+
 /**
  * display_read_timing() - Read timing information
  *