msm: msi: add Synopsys MSI support

In addition to QGIC MSI, Synopsys PCIe core support MSI
internally. Add the support to configure, enable, and
maintain interrupts for Synopsys MSI.

Change-Id: Ie5ab408123d12e8ee4ad3685d9a5bfb537e2e879
Signed-off-by: Tony Truong <truong@codeaurora.org>