powerpc/8xxx: Update device tree bus probe for new RapidIO node binding

Update of_platform_bus_probe() RapidIO node to be compitable with
new RapidIO dts compatible property.

Signed-off-by: Kai Jiang <Kai.Jiang@freescale.com>
Acked-by: Alexandre Bounine <alexandre.bounine@idt.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>