parisc: document light weight syscall ABI

Document the LWS ABI including implementation notes for
userspace, and comment cleanup.

Remove extraneous .align 16 after lws_lock_start.

Signed-off-by: Carlos O'Donell <carlos@systemhalted.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>