media: mtk-vcodec: fix build breakage when one of VPU or SCP is enabled

The addition of MT8183 support added a dependency on the SCP remoteproc
module. However the initial patch used the "select" Kconfig directive,
which may result in the SCP module to not be compiled if remoteproc was
disabled. In such a case, mtk-vcodec would try to link against
non-existent SCP symbols. "select" was clearly misused here as explained
in kconfig-language.txt.

Replace this by a "depends" directive on at least one of the VPU and
SCP modules, to allow the driver to be compiled as long as one of these
is enabled, and adapt the code to support this new scenario.

Also adapt the Kconfig text to explain the extra requirements for MT8173
and MT8183.

Reported-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: bf1d556a

 ("media: mtk-vcodec: abstract firmware interface")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>