media: rkisp1: Move debugfs code to a separate file

To avoid cluttering the main rkisp1-dev.c driver file, move debugfs code
to a separate source file. This prepares for extensions to the debugfs
infrastructure.

While at it, add a missing forward declaration for struct dentry in
rkisp1-common.h to avoid depending on indirect includes.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>