ARM: ux500: Match Power-On-Key MFD and DTS compatible strings

The compatible string found in the Power-On-Key's MFD cell matches
the device name provided in the driver. Keeping this naming convention
seems like a good idea, so we're changing the one found in the DTS
file to match.
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

arch/arm/boot/dts/dbx5x0.dtsi

@@ -374,7 +374,7 @@ ab8500-usb {
 				};
 
 				ab8500-ponkey {
-					compatible = "stericsson,ab8500-ponkey";
+					compatible = "stericsson,ab8500-poweron-key";
 					interrupts = <6 0x4
 						      7 0x4>;
 					interrupt-names = "ONKEY_DBF", "ONKEY_DBR";