drivers: espi: xec: mec172x: Fix compilation when OOB RX async is enabled

Correct MEC172x OOB register access that causes compilation error,
Use device-tree-based register access instead of HAL access
This occurs whenever CONFIG_ESPI_OOB_CHANNEL_RX_ASYNC is
enabled.

Signed-off-by: Jose Alberto Meza <jose.a.meza.arellano@intel.com>