MIPS: cavium-octeon: fix early boot hang on EBH5600 board

The boot hangs early on EBH5600 board when octeon_fdt_pip_iface() is
trying enumerate a non-existant interface. The actual hang happens in
cvmx_helper_interface_get_mode():

	mode.u64 = cvmx_read_csr(CVMX_GMXX_INF_MODE(interface));

when interface == 4. We can avoid this situation by first checking that
the interface exists in the DTB.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nsn.com>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6101/