powerpc: Update obsolete comments in setup_32.c about entry conditions

early_init() is called in-place before kernel relocation and using
whatever MMU setup exists at the point the kernel is entered.

machine_init() is called after relocation and after some initial
mapping of PAGE_OFFSET has been established (typically using BATs
on 6xx/7xx/7xxx processors or some form of bolted TLB on others).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>