ARM64: dts: meson: fix clock source of the pclk for UART_AO

>From the hardware perspective, the actual pclk of the AO uarts
is the corresponding clkc_ao uart gate, not the main clock controller clk81.
This was not problem so far, because the uart_gate had
the CLK_IGNORE_UNUSED flag, which kept the gate open.

We plan to remove the CLK_IGNORE_UNUSED flag in another patch,
but before doing that, we need to fix the clock in the DTS file.

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