powerpc/8xx: Reverse order entries are written by __set_pte_at()

At the time being, with 16k pages __set_pte_at() writes table entries
in reverse order:

 294:	91 49 00 0c 	stw     r10,12(r9)
 298:	91 49 00 08 	stw     r10,8(r9)
 29c:	91 49 00 04 	stw     r10,4(r9)
 2a0:	91 49 00 00 	stw     r10,0(r9)

Allthough there should be no impact at all as it stays in a single
cacheline, reverse the writing in a more natural order.

 288:	91 49 00 0c 	stw     r10,0(r9)
 28c:	91 49 00 08 	stw     r10,4(r9)
 290:	91 49 00 04 	stw     r10,8(r9)
 294:	91 49 00 00 	stw     r10,12(r9)

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/67c3b5d44edfec054234ea9b4d05fc4b4f7f8a0e.1664346554.git.christophe.leroy@csgroup.eu