MIPS: Lantiq: Fix interface clock and PCI control register offset

The XRX200 based SoC have a different register offset for the interface
clock and PCI control registers. This patch detects the SoC and sets the
register offset at runtime. This make PCI work on the VR9 SoC.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4113/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>