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>