ASoC: atmel-classd: remove aclk clock

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.

Thus, remove all mentions to aclk in classd driver and update macros and
variable names.

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