clk: imx: imx8mm: correct audio_pll2_clk to audio_pll2_out

There is no audio_pll2_clk registered, it should be audio_pll2_out.

Cc: <stable@vger.kernel.org>
Fixes: ba5625c3 ("clk: imx: Add clock driver support for imx8mm")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>