spi: omap2-mcspi: add support for pm_runtime autosuspend

Adds support for configuring the omap2-mcspi driver use autosuspend for
runtime power management. This can reduce the latency in starting an
spi transfer by not suspending the device immediately following
completion of a transfer. If another transfer then takes place before
the autosuspend timeout (2 secs), the call to resume the device can
return immediately saving some save/ restore cycles.

Acked-by: Govindraj.R <govindraj.raja@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>