spi: mpc512x: cleanup clock API use

cleanup the MPC512x SoC's SPI master's use of the clock API
- get, prepare, and enable the MCLK during probe; disable, unprepare and
  put the MCLK upon remove; hold a reference to the clock over the
  period of use
- fetch MCLK rate (reference) once during probe and slightly reword BCLK
  (bitrate) determination to reduce redundancy as well as to not exceed
  the maximum text line length
- stick with the PPC_CLOCK 'psc%d_mclk' name for clock lookup, only
  switch to a fixed string later after device tree based clock lookup
  will have become available

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