msm: pcie: add support to handle aggregated interrupts

Not all targets will have a dedicated line for each PCIe
interrupt. On these targets, some PCIe interrupts will be
aggregated into one line. Thus, add support to handle
aggregated interrupts.

Change-Id: I4f5be73718d4a4ae8a3de142579f24a7113fe086
Signed-off-by: Tony Truong <truong@codeaurora.org>