drivers: espi: xec: support for eSPI slaves with long initializations

Add Kconfig switch to disable automatic eSPI slave boot
acknowledgement.
This allows to perform lenghty operations before continue any eSPI
handshake with eSPI master.

Required for eSPI SAF boot configuration.

Signed-off-by: Jose Alberto Meza <jose.a.meza.arellano@intel.com>