ARM64: dts: meson-gxl: fix Mali GPU compatible string

meson-gxl-mali.dtsi is only used on GXL SoCs. Thus it should use the GXL
specific compatible string instead of the GXBB one.
For now this is purely cosmetic since the (out-of-tree) lima driver for
this GPU currently uses the "arm,mali-450" match instead of the SoC
specific one. However, update the .dts to match the documentation since
this driver behavior might change in the future.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>