media: hantro: Make struct hantro_variant.init() optional

The hantro_variant.init() function is there for platforms
to perform hardware-specific initialization, such as
clock rate bumping.

Not all platforms require it, so make it optional.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>