arm64: dts: exynos: disable non-working GPU on Exynos7 Espresso

The Panfrost GPU drivers require clock but such was not provided in
Exynos7 DTSI.  The CMU_G3D clock controller was not upstreamed, thus
consider GPU as non-working and simply disable it to silence warnings
like:

  exynos7-espresso.dtb: gpu@14ac0000: 'clocks' is a required property

Link: https://lore.kernel.org/r/20230120173116.341270-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>