arm64: dts: meson: axg: fix audio fifo reg size

The register region size initially is too small to access all
the fifo registers.

Fixes: f2b8f6a9 ("arm64: dts: meson-axg: add audio fifos")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>