ARM: dts: stm32: YAML validation fails for STM32MP15 ST Boards

"make dtbs_check" gives following output :
stm32mp157x-xxx.dt.yaml: gpu@59000000: 'contiguous-area' does not match
any of the regexes: 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/gpu/vivante,gc.yaml

Solve this trouble for STM32MPU Boards :
- stm32mp157c-ed1
- stm32mp157x-dkx

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