media: ddbridge: const'ify all ddb_info, ddb_regmap et al

All data is accessed RO, so mark everything const. Some vars in several
functions aswell as function signatures also require the const keyword
now, they're also added by this commit.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>