net: ipa: define IPA v3.1 GSI interrupt register offsets

Add definitions of the offsets for IRQ-related GSI registers.  Use
gsi_reg() rather than the corresponding GSI_CNTXT_*_OFFSET() macros
to get the offsets for these registers, and get rid of the macros.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>