ASoC: atmel-classd: remove aclk clock from DT binding

Since gclk (generated-clk) is now able to determine the rate of the
audio_pll, there is no need for classd to have a direct phandle to the
audio_pll while already having a phandle to gclk.

This binding is used by no board in mainline so it is safe to be
modified.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>