ARM: ux500: Configure the PRCMU Timer for db8500 based devices in DT

This patch adds the information required for successful
registration of the PRCMU timer 4 (clocksource) driver to the
db8500 Device Tree include file.

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