ARM: dts: stm32: YAML validation fails for Argon Boards

"make dtbs_check" gives following output :
stm32mp157c-emstamp-argon.dtb: gpu@59000000: 'contiguous-area' does not match
any of the regexes: 'pinctrl-[0-9]+'
From schema: Documentation/devicetree/bindings/gpu/vivante,gc.yaml

Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>