ARM: ux500: Identify the PRCMU as an interrupt controller

We're just about to provide the DB8500-PRCMU with its own IRQ domain,
so that its subordinate drivers can use it as an interrupt controller.
It's obligatory for all IRQ controllers to reference themselves as
such from its own node in Device Tree. This patch does just that.

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