net: ipa: define fields for event-ring related registers

Define field IDs for the EV_CH_E_CNTXT_0 and EV_CH_E_CNTXT_8 GSI
registers, and populate the register definition files accordingly.
Use the reg_*() functions to access field values for those regiters,
and get rid of the previous field definition constants.

The remaining EV_CH_E_CNTXT_* registers are written with full 32-bit
values (and have no fields).

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>