nios2: add arch/nios2/soc/<soc>/include to linker include path

Linker scripts for Nios II will also need these headers as it
specifies the device's physical layout and various important
vector addresses.

Change-Id: Ie9efaf19e53d2493eed7b9783052393d7ea9dd0f
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>