accel/ivpu: Show NPU frequency in sysfs

Add sysfs files that show maximum and current
frequency of the NPU's data processing unit.
New sysfs entries:
- npu_max_frequency_mhz
- npu_current_frequency_mhz

Signed-off-by: Andrzej Kacprowski <Andrzej.Kacprowski@intel.com>
Signed-off-by: Maciej Falkowski <maciej.falkowski@linux.intel.com>
Reviewed-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Link: https://lore.kernel.org/r/20250401155912.4049340-3-maciej.falkowski@linux.intel.com