drm/fsl-dcu: fix endian issue when using clk_register_divider

Since using clk_register_divider to setup the pixel clock, regmap
is no longer used. Regmap did take care of DCU using different
endianness. Check endianness using the device-tree property
"big-endian" to determine the location of DIV_RATIO.

Cc: stable@vger.kernel.org
Fixes: 2d701449 ("drm/fsl-dcu: use common clock framework for pixel clock divider")
Reported-by: Meng Yi <meng.yi@nxp.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Tested-by: Meng Yi <meng.yi@nxp.com>