msm: msi: add support for Snyopsys MSI IRQ mask/unmask and enable/disable

Synopsys MSIs supports many virq to one hwirq. Therefore,
the hwirq should not be masked/unmasked when clients call
mask/unmask for one of their virq. Other virqs can still
be enabled. Add support to handle masking/unmasking of MSIs
at PCIe core.

Change-Id: Idcfdb1f34e3dc496c25513de3e67834018eee91b
Signed-off-by: Tony Truong <truong@codeaurora.org>