media: pixfmt-tch-td16/tu16.rst: document that this is little endian

Testing with the rmi_f54 driver on the Lenovo X1 Carbon 6th gen
laptop showed that the data is in little endian format. Update
the documentation accordingly.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>