nios2: add _nios2_reg_write/read functions

The technical manuals and example HAL code frequently refer to
register bank numbers from some base address. Add these helper
functions to read and write registers correctly using this
notation.

Change-Id: Ia082f5cc89081fcea2cb6ad8204c1b9b2650d3fd
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>