spmi: spmi-pmic-arb: make interrupt support optional

Make the support of PMIC peripheral interrupts optional for
spmi-pmic-arb devices.  This is useful in situations where
SPMI address mapping is required without the need for IRQ
support.

Change-Id: Ifdacf9b992d80c8ab561917fc4eb4816b8b9c8ff
Signed-off-by: David Collins <collinsd@codeaurora.org>