spi/pl022: add support for pm_runtime autosuspend

Adds support for configuring the spi bus to 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, the call to resume the device can return
immediately rather than needing to risk sleeping in order to resume
the device.

Reviewed-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Chris Blair <chris.blair@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>