boards: phytec: Remove am62x from phyboard_lyra_am62x

Both phyBOARD-Lyra AM62x boards were called
phyboard_lyra_am62x/am6234/a53 or phyboard_lyra_am62x/am6234/m4.
Since the AM62x SOC part number is now included in the board name
as am6234, there is no need to have am62x in the board name itself.

Moreover, PHYTEC also has an AM62Ax SOM which comes with the same
carrier-board. Integration of this kit would be easier later.

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