arm64: dts: imx8qm: Fix VPU core alias name

arm64: dts: imx8qm: Fix VPU core alias name

Alias names use dashes instead of underscores, fix this. Silences also
dtbs_check warning:
imx8qxp-tqma8xqp-mba8xx.dtb: aliases: 'vpu_core0', 'vpu_core1', 'vpu_core2'
 do not match any of the regexes: '^[a-z][a-z0-9\\-]*$', 'pinctrl-[0-9]+'
from schema $id: http://devicetree.org/schemas/aliases.yaml#

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>