arm64: dts: imx8mp: Enable SAI audio on MX8MP DHCOM PDK2 and PDK3

Add SAI I2S and audio bindings on MX8MP DHCOM PDK2 and PDK3.

The VDDA is supplied from on-carrier-board regulator, the VDDIO
is supplied from always-on on-SoM regulator. Except for different
I2C bus used to connect the codec, the implementation is virtually
identical on both carrier boards.

Align regulator-avdd name to regulator-3p3vdd on PDK3, since this
is the VDDA supply and it is the same on both carrier boards.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>