backlight: lp855x_bl: support new LP8555 device

LP8555 is one of the LP855x family devices.

This device needs pre_init_device() and post_init_device() driver
structure.  It's same as LP8557, so the device configuration code is
shared with LP8557.  Backlight outputs are generated from dual DC-DC boost
converters.  It's configurable EPROM settings which are defined in the
platform data.

Driver documentation and device tree bindings are updated.

Signed-off-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/devicetree/bindings/video/backlight/lp855x.txt

@@ -2,7 +2,7 @@ lp855x bindings
 
 Required properties:
   - compatible: "ti,lp8550", "ti,lp8551", "ti,lp8552", "ti,lp8553",
-                "ti,lp8556", "ti,lp8557"
+                "ti,lp8555", "ti,lp8556", "ti,lp8557"
   - reg: I2C slave address (u8)
   - dev-ctrl: Value of DEVICE CONTROL register (u8). It depends on the device.
 
@@ -15,6 +15,33 @@ Optional properties:
 
 Example:
 
+	/* LP8555 */
+	backlight@2c {
+		compatible = "ti,lp8555";
+		reg = <0x2c>;
+
+		dev-ctrl = /bits/ 8 <0x00>;
+		pwm-period = <10000>;
+
+		/* 4V OV, 4 output LED0 string enabled */
+		rom_14h {
+			rom-addr = /bits/ 8 <0x14>;
+			rom-val = /bits/ 8 <0xcf>;
+		};
+
+		/* Heavy smoothing, 24ms ramp time step */
+		rom_15h {
+			rom-addr = /bits/ 8 <0x15>;
+			rom-val = /bits/ 8 <0xc7>;
+		};
+
+		/* 4 output LED1 string enabled */
+		rom_19h {
+			rom-addr = /bits/ 8 <0x19>;
+			rom-val = /bits/ 8 <0x0f>;
+		};
+	};
+
 	/* LP8556 */
 	backlight@2c {
 		compatible = "ti,lp8556";