spmi: pmic_arb: remove struct pmic_arb_irq_spec

struct pmic_arb_irq_spec serves no real purpose. It is used only
in the translate function and the code is much cleaner without it.

Change-Id: I4daae954b8e01b33a9c7f5a343a31c15b4787382
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>