arch: arm: dts: k3-am642-phyboard-electra: Add fixed partitions

Add a fixed partitions node to the AM64x device tree so that it can
be used to fixup the Linux device tree.

Signed-off-by: Nathan Morrisson <nmorrisson@phytec.com>
Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Reviewed-by: Daniel Schultz <d.schultz@phytec.de>