arm64: dts: meson: Describe G12b GPU as coherent

According to a downstream commit I found in the Khadas vendor kernel,
the GPU on G12b is wired up for ACE-lite, so (now that Panfrost knows
how to handle this properly) we should describe it as such. Otherwise
the mismatch leads to all manner of fun with mismatched attributes and
inadvertently snooping stale data from caches, which would account for
at least some of the brokenness observed on this platform.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/765446e529e50b304af63432da7836c4d31eb8d4.1600780574.git.robin.murphy@arm.com