octeon: Add and set CPU type Octeon+ as default

octeon: Add and set CPU type Octeon+ as default

The lowest CPU type used by supported Octeon platform
is Octeon+ (EdgeRouter Lite) while EdgeRouter Pro/ER-8 uses
Octeon II which is backwards compatible with Octeon+.

Sources:
https://community.ubnt.com/t5/EdgeRouter/EdgeRouter-Pro-CPU/td-p/654599
https://www.cavium.com/octeon-II-CN68XX.html
"OCTEON II family is fully software compatible with the widely-adopted
OCTEON Plus family"

Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>
(backported from c6e02b49f65cb4eff624a0831d3db265b3fadd2a)