drivers/espi: Add support for ESPI_PERIPHERAL_HOST_IO_PVT2 & PVT3

Add support the host I/O over eSPI peripheral channel for private
channel2 and channel3.

The default port number of ESPI_PERIPHERAL_HOST_IO_PVT2_PORT_NUM and
ESPI_PERIPHERAL_HOST_IO_PVT3_PORT_NUM are 0x68 and 0x70.

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