powerpc: Add comment explaining the purpose of setup_kdump_trampoline()

Anything in early_setup() needs to be justified to be there, in
this case, we need the trampolines before we can take exceptions
and thus before we turn on the MMU.

Also remove a pretty meaningless and misplaced debug message

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