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>