mfd: Provide the PRCMU with its own IRQ domain

The PRCMU has its own USB, Thermal, GPIO, Modem, HSI and RTC drivers,
amongst other things. This patch allows those subordinate devices to
use it as an interrupt controller as and when they are DT enabled.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>