drivers: espi: Microchip eSPI add SAF support

Add ESPI SAF features to the Microchip eSPI driver as
a separate file. ESPI SAF depends upon the core eSPI
driver adding the ability to attach the system SPI
flash to the EC eSPI endpoint instead of the host
eSPI controller.

Signed-off-by: Scott Worley <scott.worley@microchip.com>