ARM: dts: ux500: Use "arm,pl031" compatible for PL031

ARM: dts: ux500: Use "arm,pl031" compatible for PL031

The Ux500 device tree uses "arm,rtc-pl031" as compatible for PL031.
All other boards in Linux describe it using "arm,pl031" instead.
This works because the compatible is not actually used in Linux:
AMBA devices get probed based on "arm,primecell" and their peripheral ID.

Nevertheless, some other projects (e.g. U-Boot) rely on the compatible
to probe the device with the correct driver. Those will look for
"arm,pl031" instead of "arm,rtc-pl031", preventing the RTC from being
probed.

Change it to "arm,pl031" to match all other boards.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20191124205110.48031-1-stephan@gerhold.net
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>