parisc: document the parisc gateway page

Include some documentation about how the parisc gateway page technically
works and how it is used from userspace.

James Bottomley is the original author of this description and it was
copied here out of an email thread from Apr 12 2013 titled:
man2 : syscall.2 : document syscall calling conventions

CC: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Helge Deller <deller@gmx.de>