spi/pl022: disable core voltage when idle

This utilizes the new core voltage switch to power off the PL022
core voltage when it's not in use transmitting packets, if and
only if a core voltage switch is available.

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>