net: ipa: define IPA v3.1 GSI event ring register offsets

Add definitions of the offsets and strides for registers whose
offset depends on an event ring ID, and use gsi_reg() and its
returned value to determine offsets for these registers.  Get
rid of the corresponding GSI_EV_CH_E_*_OFFSET() macros.

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