media: imx274: add support for sensor mode6, 1280x540

Add support for the mode 6 for the sensor, this mode uses
3/8 subsampling and 3 horizontal binning.
Aspect ratio is changed.
Split the bin_ratio variable into two parts, one for
width and one for height, as the ratio is no longer preserved
when doing subsampling in this mode.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>