fbdev: sh_mobile_lcdc: Implement overlays support

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Documentation/ABI/testing/sysfs-devices-platform-sh_mobile_lcdc_fb

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+What:		/sys/devices/platform/sh_mobile_lcdc_fb.[0-3]/graphics/fb[0-9]/ovl_alpha
+Date:		May 2012
+Contact:	Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+Description:
+		This file is only available on fb[0-9] devices corresponding
+		to overlay planes.
+
+		Stores the alpha blending value for the overlay. Values range
+		from 0 (transparent) to 255 (opaque). The value is ignored if
+		the mode is not set to Alpha Blending.
+
+What:		/sys/devices/platform/sh_mobile_lcdc_fb.[0-3]/graphics/fb[0-9]/ovl_mode
+Date:		May 2012
+Contact:	Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+Description:
+		This file is only available on fb[0-9] devices corresponding
+		to overlay planes.
+
+		Selects the composition mode for the overlay. Possible values
+		are
+
+		0 - Alpha Blending
+		1 - ROP3
+
+What:		/sys/devices/platform/sh_mobile_lcdc_fb.[0-3]/graphics/fb[0-9]/ovl_position
+Date:		May 2012
+Contact:	Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+Description:
+		This file is only available on fb[0-9] devices corresponding
+		to overlay planes.
+
+		Stores the x,y overlay position on the display in pixels. The
+		position format is `[0-9]+,[0-9]+'.
+
+What:		/sys/devices/platform/sh_mobile_lcdc_fb.[0-3]/graphics/fb[0-9]/ovl_rop3
+Date:		May 2012
+Contact:	Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+Description:
+		This file is only available on fb[0-9] devices corresponding
+		to overlay planes.
+
+		Stores the raster operation (ROP3) for the overlay. Values
+		range from 0 to 255. The value is ignored if the mode is not
+		set to ROP3.