powerpc/6xx: Store PGDIR physical address in a SPRG

Use SPRN_SPRG2 to store the current thread PGDIR and
avoid reading thread_struct.pgdir at every TLB miss.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>