media: staging: rkisp1: group declaration of similar functions together

In file rkisp1-common.h, group declaration of register/unregister
functions together and group other functions together to make
the code easier to read.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>