arch/mips: add (Marvell) Octeon III processor

arch/mips: add (Marvell) Octeon III processor

The compiler recognizes a specific 'march' value for Octeon III processors,
so create a 'Target Architecture Variant' entry for it in the target menu.

Note: support for '-march=octeon3' was added in gcc 5.x. However, the
official compiler provided by Marvell (Cavium Networks) uses gcc 4.7.x (and
supports -march=octeon3 via their own modifications). For this reason, no
line 'select BR2_ARCH_NEEDS_GCC_AT_LEAST_5' is added.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>