msm: irqs: Factor out PPI/SPI offset definitions

The definitions of GIC_PPI_START and GIC_SPI_START are the
same on all targets, so move them into the common IRQ file
in preparation for removing per-target IRQ files.

Change-Id: I793b1e4ca79633956fa10c33da9f736b5d730751
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>