xscale minor cleanup

Add a header comment referencing useful XScale specs.
Make most data static, and the tables readonly.
Scrub extra blank lines.
Return fault codes from one routine.
Remove a needless NOP methood.

(BUGFIX) When we update R0, mark R0 as dirty/valid ... not R15/PC!

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