xtensa: intel_adsp: use cavs_ipc driver only if enabled in Kconfig

Commit bdce0a57 ("soc/intel_adsp: Add a cavs_ipc driver to manage
host IPC") added a new driver for cAVS IPC. Although patch included the
CONFIG_CAVS_IPC option, in practise this had no effect and instead
driver was enabled whenever device tree included IPC hardware. This
caused IPC errors in SOF application builds where two drivers, one out
of tree in SOF and one in Zephyr, initialize the same hardware.

Fix the issue by enabling the driver only when CONFIG_CAVS_IPC is
enabled.

BugLink: https://github.com/thesofproject/sof/issues/5477


Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>