emul: espi: Add support for eSPI emulators

Add an emulation controller which routes eSPI traffic to attached
emulators depending on the selected chip(mostly host).
This allows drivers for eSPI peripherals to be tested on systems
that don't have that peripheral attached, with the emulator handling
the eSPI traffic.

Signed-off-by: Dawid Niedzwiecki <dn@semihalf.com>