docs: powerpc: Clarify book3s/32 MMU families

Documentation wrongly tells that book3s/32 CPU have hash MMU.

603 and e300 core only have software loaded TLB.

755, 7450 family and e600 core have both hash MMU and software loaded
TLB. This can be selected by setting a bit in HID2 (755) or
HID0 (others). At the time being this is not supported by the kernel.

Make this explicit in the documentation.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/261923c075d1cb49d02493685e8585d4ea2a5197.1593698951.git.christophe.leroy@csgroup.eu