mips: octeon: lowlevel_init.S: Add NMI handling code for SMP Linux booting

This patch adds the necessary lowlevel init code, to enable SMP Linux
booting. This code will be used with the platform specific Octeon Linux
boot command "bootoctlinux", which starts a configurable number of cores
into Linux.

Additionally some erratas and lowlevel register initializations are
copied from the original Cavium / Marvell U-Boot source code, enabling
booting into the Linux kernel.

Signed-off-by: Stefan Roese <sr@denx.de>