spi: meson-axg: update compatible string for the Meson-AXG

Update the compatbile string to support Meson-AXG SoCs.

Signed-off-by: Sunny Luo <sunny.luo@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>