mfd/db8500-prcmu: use the irq_domain_add_simple()

To be able to use SPARSE_IRQ while yet not using device tree,
we need to use irq_domain_add_simple() that will allocate
descriptors for the IRQs in the non-DT case, and fall back
to using the linear irqdomain in the DT case.

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