powerpc: Fix OF parsing of 64 bits PCI addresses

The OF parsing code for PCI addresses isn't always treating properly
the address space indication 0b11 (ie. 0x3) as meaning 64 bits
memory space.

This means that it fails to parse addresses for PCI BARs that have
this encoding set by the firmware, which happens on some SLOF
versions and breaks offb palette handling on Powerstation.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>