arm64: dts: phycore-imx8mm: Label typo-fix of VPU

Corrected the label of the VPU regulator node (buck 3)
from reg_vdd_gpu to reg_vdd_vpu.

Fixes: ae6847f2 ("arm64: dts: freescale: Add phyBOARD-Polis-i.MX8MM support")
Signed-off-by: Yashwanth Varakala <y.varakala@phytec.de>
Signed-off-by: Cem Tenruh <c.tenruh@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>