MIPS: BCM63xx: Add SMP support to prom.c

This involves two changes to the BSP code:

1) register_smp_ops() for BMIPS SMP

2) The CPU1 boot vector on some of the BCM63xx platforms conflicts with
the special interrupt vector (IV).  Move it to 0x8000_0380 at boot time,
to resolve the conflict.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
[jogo@openwrt.org: moved SMP ops registration into ifdef guard,
 changed ifdef guards to if (IS_ENABLED())]
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: John Crispin <blogic@openwrt.org>
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Patchwork: https://patchwork.linux-mips.org/patch/5489/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>