powerpc/8xx: Handle PAGE_USER via APG bits

Use of APG for handling PAGE_USER.

All pages PP exec bits are set to either 000 or 011, which means
respectively RW for Supervisor and no access for User, or RO for
Supervisor and no access for user.

Then we use the APG to say whether accesses are according to
Page rules or "all Supervisor" rules (Access to all)

Therefore, we define 2 APG groups corresponding to _PAGE_USER.
Mx_AP are initialised as follows:
GP0 => No user => 01 (all accesses performed according
				to page definition)
GP1 => User => 00 (all accesses performed as supervisor
                                according to page definition)

This removes the special 8xx handling in pte_update()

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>