net: ipa: remove a remoteproc dependency

The IPA driver currently requires a DT property to be defined whose
value is the phandle for the modem subsystem.  This was needed to
look up a remoteproc structure pointer used when registering for
notifications in the original IPA notification mechanism.

Remoteproc provides a more generic SSR notifier system, and the IPA
driver switched over to it last summer, but this remoteproc phandle
dependency was not removed at that time.

Get rid of the IPA remoteproc pointer and stop requiring the phandle
be specified.

This avoids a link error (rproc_put() not defined) for certain
configurations.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>