dt: Remove obsolete description of powerpc boot interface

32 and 64 bit powerpc support has been merged for a while now, but
the booting-without-of.txt document still describes 32 bit as not
supporting multiplatform, which is no longer true.  This patch fixes
the documentation.

Also remove references to powerpc-specific details outside of section
I in preparation to add details for other architectures.

v3: cleaned up a lot more powerpc-isms and updated text to reflect current
    usage conventions.
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>