Add driver for SAMA7G5's PDMC

Merge series from Codrin Ciubotariu <codrin.ciubotariu@microchip.com>:

This patch series adds support for Pulse Density Microphone Controller
(PDMC), present on Microchip's SAMA7G5.
The PDMC interfaces up to 4 digital microphones having Pulse Density
Modulated (PDM) outputs. It generates a single clock line and samples 1 or
2 data lines. The signal path includes an audio grade programmable
decimation filter and outputs 24-bit audio words.
The source of each channel can be independently defined as PDMC_DS0 or
PDMC_DS1, sampled at the rising or falling edge of PDMC_CLK.

The patch series starts with a fix on the ASoC DMA engine support. Then
continues with the bindings and the driver of PDMC. It is followed by the
DT nodes for SAMA7G5 and SAMA7G5-EK. In the end, the drivers for PDMC
and PDM microphones are enabled in sama7_defconfig.