ARM: ux500: Enable the AB8500 RTC for all DT:ed DB8500 based devices

Here we add a node for the AB8500 Real Time Clock in all devices
supporting the DB8500. The AB8500 RTC driver makes use of named
interrupts we provide support for this too.

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