arm: mach-k3: Rename SOC_K3_AM6 to SOC_K3_AM654

The first AM6x device was the AM654x, but being the first we named it
just AM6, since more devices have come out with this same prefix we
should switch it to the normal convention of using the full name of the
first compatibility device the series. This makes what device we are
talking about more clear and matches all the K3 devices added since.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>