mips: start.S: Add Octeon boot header compatibility

mips: start.S: Add Octeon boot header compatibility

Octeon has a specific boot header, when booted via SPI NOR, NAND or MMC.
Here the only 2 instructions are allowed in the first few bytes of the
image. And these instructions need to be one branch and a nop. This
patch adds the necessary nop after the nop, to that the common MIPS
image is compatible with this Octeon header.

The tool to patch the Octeon boot header into the image will be send in
a follow-up patch.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Aaron Williams <awilliams@marvell.com>
Cc: Chandrakala Chavva <cchavva@marvell.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>