drivers/espi: ite: Use structured evt_data for PVT, PVT2, and PVT3 events

Replace direct assignment of evt.evt_data with structured access via
espi_evt_data_pvt. This change separates the incoming evt_data into
type and data fields.

This update alse includes support for handling PVT, PVT2, and PVT3
events using the structured event data.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>