media: staging: media: imx: imx7_mipi_csis: Add timings override through debugfs

Add two debugfs files, ths_settle and tclk_settle, to allow overriding
the corresponding timing parameters for test purpose.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com> # On i.MX8MP
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>