net: ipa: support different event ring encoding

Starting with IPA v5.0, a channel's event ring index is encoded in
a field in the CH_C_CNTXT_1 GSI register rather than CH_C_CNTXT_0.
Define a new field ID for the former register and encode the event
ring in the appropriate register.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>