ARM: dts: stm32: add audio codec support on stm32mp157a-dk1 board

Add support of Cirrus cs42l51 audio codec on stm32mp157a-dk1 board.
Configuration overview:
- SAI2A is the CPU interface used for the codec audio playback
- SAI2B is the CPU interface used for the codec audio record
- SAI2A is configured as a clock provider for the audio codec
- SAI2A&B are configured as slave of the audio codec
- SAI2A&B share the same interface of the audio codec

Note:
In master mode, cs42l51 audio codec provides a bitclock
at 64 x FS, regardless of data width. This means that
slot width is always 32 bits.
Set slot width to 32 bits and slot number to 2
in SAI2A&B endpoint nodes, to match this constraint.
dai-tdm-slot-num and dai-tdm-slot-width properties are used here,
assuming that i2s is a special case of tdm, where slot number is 2.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>