powerpc/mm: Fix output of total_ram.

On 32bit platforms that support >= 4GB memory total_ram was truncated.
This creates a confusing printk:
	Top of RAM: 0x100000000, Total RAM: 0x0
Fix that:
	Top of RAM: 0x100000000, Total RAM: 0x100000000

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>