powerpc: Fix warning in setup_64.c when CONFIG_RELOCATABLE=y

When CONFIG_RELOCATABLE is enabled, PHYSICAL_START is actually a
variable of type phys_addr_t. That means to print it we need to
cast to unsigned long long and use llx.

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