On multi-chip boards, the first core on slave SoCs may take much
more time to wakeup. Add code to wait for the core to come up before
proceeding with the rest of the boot up.

Update xlp_wakeup_core to also skip the boot node and the boot CPU
initialization which is already complete.
Signed-off-by: NJayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/4783/Signed-off-by: NJohn Crispin <blogic@openwrt.org>