mxc_spi: copy inactive polarity

Use the clock polarity as inactive polarity as well.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
mxc_spi: Keep ecspi enabled

The ECSPI must be enabled before changing other
registers. The comment in the code was wrong.
This is needed for SPI_MODE3 to work correctly.
Otherwise, the clock will start out as low after
the chip select GPIO is activated.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
add enable_spi_clk
add support for SPI initialization of display panels