media: allegro: add HEVC NAL unit generator

When encoding a video as HEVC, the allegro driver needs to generate the
Non-VCL NAL units for HEVC. Do the same as for H.264 and add a module
that takes C structs for the VPS/SPS/PPS and encodes the fields as RBPS
as specified by "ITU-T Rec. H.265 (02/2018) high efficiency video
coding".

[hverkuil: add missing static for nal_hevc_write/read_start_code_prefix]
[hverkuil: fix typo: pps -> vps in nal_hevc_write_vps]

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>