spi: spi-mxc: implement clk control for ECSPI to fix SPI_MODE_3

SPI_MODE_3 requires clk high when inactive. The SCLK_CTL
field of the config reg was not configured in case of CPOL.
Fix configuration so that SPI_MODE_3 which uses CPOL configures
the clk line to be high in inactive state.

Signed-off-by: Markus Niebel <Markus.Niebel@tqs.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>