phycore-am64x: Migrate to OF_UPSTREAM

The phycore-am64x can be migrated to OF_UPSTREAM.
Drop redundant device tree files, update MAINTAINERS and
defconfig's DEFAULT_DEVICE_TREE for ti vendor prefix accordingly.
While at it, drop the redundant definitions of AM642_PHYBOARD_ELECTRA_DTB
& SPL_AM642_PHYBOARD_ELECTRA_DTB from the binman dtsi file.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>