arm: mach-k3: Move K3 common schema.yaml out of board directory

This file is common for all K3, move it out of board/ directory and
into mach-k3. As we need to change the path in k3-binman.dtsi let's
take this opportunity to switch to absolute paths which makes adding
non-TI boards (like Toradex Verdin) not need to override these paths.

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