powerpc/ptdump: drop dummy KERN_VIRT_START on PPC32

PPC32 doesn't have KERN_VIRT_START. Make PAGE_OFFSET the
default starting address for the dump, and drop the dummy
definition of KERN_VIRT_START. Only use KERN_VIRT_START for
non radix PPC64.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/334632b1df4775b0ccf3bdc8d6b201d14e3daedd.1565786091.git.christophe.leroy@c-s.fr