Start switching MMU handling over to a more sensible scheme.
Having an mmu() method enables MMU-aware behaviors.  Not having
one kicks in simpler ones, with no distinction between virtual
and physical addresses.

Currently only a handful of targets have methods to read/write
physical memory:  just arm720, arm920, and arm926.  They should
all initialize OK now, but the arm*20 parts don't do the "extra"
stuff arm926 does (which should arguably be target-generic).

Also simplify how target_init() loops over all targets by making
it be a normal "for" loop, instead of scattering its three parts
to the four winds.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>