boards: phytec: Merge AM62 A53 into phyboard_lyra_am62x

phyBOARD-Lyra AM62x is the official name for our kit, which we
use to develop our reference software. This kit includes the
phyCORE-AM62x with TI's AM62x SoC.

Rename the phycore_am62x_am6234_a53 board to phyboard_lyra_am62x_am6234_a53
and move all code into the phyboard_lyra_am62x directory. Thereby, the
A53 and M4 boards are located in the same directory and both named
correctly.

This is also a good chance to update documentation for both cores and
use links to our latest BSPs as well as our official AM62x Yocto machine
name for the AM62x HS-FS SOM.

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