XScale: context restore, cleanup/bugfix

XScale: context restore, cleanup/bugfix

This "loop over all registers" routine shared the same mess as
full_context() in terms of dozens of needless number_to_mode()
calls.  Fix that, and comments, with related cleanup.

The misnamed xscale_restore_context() had a related bug.  It
was restoring the *WRONG REGISTERS* ... always from whatever
the current mode was, instead of using the copy from whichever
register bank it was trying to restore.  (But it marked the
intended register as having been restored...)  Fixed that.

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