media: amphion: Fix VPU core alias name

Starting with commit f6038de2 ("arm64: dts: imx8qm: Fix VPU core
alias name") the alias for VPU cores uses dashes instead of underscores.
Adjust the alias stem accordingly. Fixes the errors:
amphion-vpu-core 2d040000.vpu-core: can't get vpu core id
amphion-vpu-core 2d050000.vpu-core: can't get vpu core id

Fixes: f6038de2 ("arm64: dts: imx8qm: Fix VPU core alias name")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>