accel/ivpu: Add dvfs_mode file to debugfs

Add new debugfs file to set dvfs_mode FW boot parameter and restart
the FW to allow experimenting with DVFS (dynamic voltage & frequency
scaling).

Signed-off-by: Tomasz Rusinowicz <tomasz.rusinowicz@intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231028133415.1169975-4-stanislaw.gruszka@linux.intel.com