board: phytec: Add phyBOARD-Electra

Add PHYTEC's phyBOARD-Electra kit with the phyCORE-AM64x
System on Module (SOM).

This board only supports the M4F core, UART and GPIO right now,
since other drivers are still missing.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>