drivers/espi: ite: Add support for ESPI_PERIPHERAL_HOST_IO_PVT

Add support the host I/O over eSPI peripheral channel for private
channel.
The default port number of ESPI_PERIPHERAL_HOST_IO_PVT_PORT_NUM
for ITE SoC is 0x68.

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