Use (un)prepare_transfer_hardware calls to set fsl-espi to
low-power idle if not in use. Reference manual states:

"The eSPI is in a idle state and consumes minimal power.
The eSPI BRG is not functioning and the input clock is disabled"
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>