Adds board definition for imx93-evk board
- Support for the Cortex-A55 core in i.MX93, support for the Cortex-M33
core is _not_ provided
- "nsh" profile is provided, this includes a minimalistic feature set
which boots the SoM to nsh console
- A bootloader is required, u-boot has been tested
- migrated /README are removed from /boards
- there are a lot of READMEs that should be further converted to rst.
At the moment they are moved to Documentation/platforms and included in rst files