ARM64: dts: meson-axg: add an 32K alt aoclk

The ao_clk81 in AO domain have two clock source,
one from a 32K alt crystal we name it as ao_alt_clk,
another is the clk81 signal from EE domain.

Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>