arm: mach-k3: j721s2: Move board selection to mach-k3

arm: mach-k3: j721s2: Move board selection to mach-k3

Currently each set of board targets from a vendor is selected inside
the board directory for that vendor. This has the problem of multiple
targets, one from each vendor, being selectable at the same time.
For instance you can select both TARGET_AM654_A53_EVM and
TARGET_IOT2050_A53 in the same build.

To fix this we need to move the target board choice to a common location
for each parent SoC selection. Do this in arch/arm/mach-k3.

Signed-off-by: Andrew Davis <afd@ti.com>