powerpc: Make boot_cpuid common between 32 and 64-bit

Move the definition to setup-common.c and set the init value
to -1 on both 32 and 64-bit (it was 0 on 64-bit).

Additionally add a check to prom.c to garantee that the init
value has been udpated after the DT scan.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>