powerpc: Print decimal values in prom_init.c

Currently we look pretty stupid when printing out a bunch of things in
prom_init.c.  eg.

  Max number of cores passed to firmware: 0x0000000000000080

So I've change this to print in decimal:

  Max number of cores passed to firmware: 128 (NR_CPUS = 256)

This required adding a prom_print_dec() function and changing some
prom_printk() calls from %x to %lu.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>