boards: mimxrt685_evk/mimxrt685s/cm33: Enable DMIC

Enable DMIC clock in soc.c - attach to chip's audio PLL. Add pinmux
definitions for the DMIC peripheral. Add nodes to SoC's device tree for
the DMIC peripheral and its audio channels. Configure the DMIC
peripheral in board's device tree to enable audio capture.

Signed-off-by: Vit Stanicek <vit.stanicek@nxp.com>