spi: mpc512x: use the SPI subsystem's message queue

the SPI subsystem recently grew support to queue messages before handing
them to the SPI master, and erroneously emitted deprecation warnings
when the SPI master's driver did not use the common logic (in fact the
master might queue messages, but implement the queue in the master
driver's source)

  [    0.823015] mpc512x-psc-spi 80011400.psc: master is unqueued, this is deprecated
  [    0.854913] mpc512x-psc-spi 80011500.psc: master is unqueued, this is deprecated

this change makes the MPC512x PSC SPI driver use the SPI subsystem's
support to queue SPI messages and removes the master driver's private
code for the queue support

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Mark Brown <broonie@linaro.org>