spi/pl022: enable runtime PM

If we're gonna use runtime PM it's a pretty good idea to actually
enable it in probe() and disable it in remove() too, so it
gets used for real. Up until now we only fooled around with the
reference count.

Cc: Vinit Shenoy <vinit.shenoy@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>