spi: mpc512x: adjust to OF based clock lookup

spi: mpc512x: adjust to OF based clock lookup

after device tree based clock lookup became available, the peripheral
driver need no longer construct clock names which include the PSC index,
remove the "psc%d_mclk" template and unconditionally use 'mclk'

acquire and release the 'ipg' clock item for register access as well

Cc: Mark Brown <broonie@kernel.org>
Cc: linux-spi@vger.kernel.org
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Acked-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>