ARM: ux500: Move rtc-pl031 registration to Device Tree when enabled

During a Device Tree boot, all probing will now be completed on parse
of the Device Tree binary. In the same patch we remove platform
registration of the Real Time Clock.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>