nios2: don't enable XIP by default

If XIP is turned on, only hardware breakpoints may be used, and
code cannot be loaded onto the device with nios2-download
or GDB 'load' command. RAM-constrained applications are free to
enable this if they need to.

Change-Id: Iee2d41f71f7ca2bc599801cf3cf0fac680273e51
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>