powerpc: Print the kernel load address at the end of prom_init()

This makes it easier to debug crashes that happen very early before
the kernel takes over Open Firmware by allowing us to relate the OF
reported crashing addresses to offsets within the kernel.

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