soc: ti: k3: Add support for AM6442

The AM64x and AM62x are both SOCs from the TI K3 family
and share common architecture designs. The M4F subsystem
is actuall identical on both SOCs.

Therefore, just add all missing CONFIGs, files, etc. to
support the AM6442x SOC.

Since MMR and RAT initialization are identical too, both
functions can be re-used. However, since they might
differ in the future, the am64x has it's own init
function.

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