nios2: provide and include soc-specific headers

BSP builds for Nios II generate a linker.h and system.h which reflects
the configuration for that CPU. This can vary depending on how the CPU
is wired up in QSYS, so it needs to be at the SOC level--we essentially
treat any given CPU configuration as a SOC in Zephyr build terms.

Include these files from <arch/cpu.h>.

Change-Id: I12f76600107fec1a14a2f9cb82b0f55915ec03a6
Origin: Altera Quartus tools, machine generated
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>