powerpc/amigaone: Limit ISA I/O range to 4k in the device tree

The kernel reserves the I/O address space from 0x0 to 0xfff for legacy
ISA devices. Change the ranges property for the PCI2ISA bridge to match
the kernels behavior, even if the ranges property isn't used for now.

Signed-off-by: Gerhard Pircher <gerhard_pircher@gmx.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>