net: ipa: add HW_PARAM_4 GSI register

Starting at IPA v5.0, the number of event rings per EE is defined
in a field in a new HW_PARAM_4 GSI register rather than HW_PARAM_2.
Define this new register and its fields, and update the code that
checks the number of rings supported by hardware to use the proper
field based on IPA version.

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