MIPS: Netlogic: replace early_init_devtree() call

The early_init_devtree() API was removed in linux-next for 3.13 with
commit "mips: use early_init_dt_scan". This causes Netlogic XLP compile
to fail:

arch/mips/netlogic/xlp/setup.c:101: undefined reference to `early_init_devtree'

Add xlp_early_init_devtree() which uses the __dt_setup_arch() to
handle early device tree related initialization to fix this.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>