spi/imx: set the inactive state of the clock according to the clock polarity

There are SPI devices which need a SPI clock with active low polarity and
high inactive state.

Add the setting of the inactive state ECSPIx_CONFIGREG:SCLK CTL
according to the clock polarity ECSPIx_CONFIGREG:SCLK POL:

DT without "spi-cpol" = 0 = clock active high polarity = inactive state low
DT with    "spi-cpol" = 1 = clock active low  polarity = inactive state high

Signed-off-by: Knut Wohlrab <knut.wohlrab@de.bosch.com>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>