media: staging: imx: fix non-static declarations

Add static keywords to fix this kind of sparse warnings:
warning: symbol 'imx_t_vcm_timing' was not declared. Should it be static?

Signed-off-by: JB Van Puyvelde <jbvanpuyvelde@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>