net: ipa: add clock reference for remoteproc SSR

The remoteproc SSR callback function for the modem requires hardware
access when handling a modem crash or shutdown.  Take and later
release an IPA clock reference in ipa_modem_crashed(), to ensure the
hardware is operational.

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