spi: spi-mxc: add defines for clk inactive state for ECSPI

Provide define for the SCLK_CTL field of the config reg of ECSPI.
While at it, oder the defines to improve readability and make
adding more defines easier.

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