spi/pl022: disable the PL022 block when unused

Make sure we clear the enable bit when the block is not used.
This will save some energy in certain hardware versions.

Reviewed-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Virupax Sadashivpetimath <virupax.sadashivpetimath@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>