greybus: spi: Add runtime_pm support

Add runtime operations to the spi driver so that the
module is woken up when an spi transfer is started
and put back to sleep when the transfer is done.

Testing Done: Let the module enter standby and initiate
an spi operation. The operation wakes up the module
and succeeds.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Alex Elder <elder@linaro.org>