powerpc/603: Switch r0 and r3 in TLB miss handlers

In preparation of next patch that will perform some additional
calculations to replace comparison, switch the use of r0 and r3
as r0 has some limitations in some instructions like 'addi/subi'.

Also remove outdated comments about the meaning of each register.
The registers are used for many things and it would be difficult
to accurately describe all things done with a given register. The
function is now small enough to get a global view without much
description.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/566af5e87685b1a85d3182549c0d520ce2d8877a.1724173828.git.christophe.leroy@csgroup.eu