arm: dts: iot2050: Add support for M.2 variant

arm: dts: iot2050: Add support for M.2 variant

Add support for the M.2 board based on the iot2050 advanced board.
The board has two m.2 connectors, one is B-keyed, the other E-keyed.
The B-key slot can connect 5G/SSD devices, and E-key can be used for
WIFI/BT devices.

This variant is covered by PG2 firmware image.

Signed-off-by: chao zeng <chao.zeng@siemens.com>
[Jan: align DT to kernel, polish wording]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>