nios2.h: cleanup API

This header was pulled in verbatim from Altera HAL and had
some style and naming issues. The inline functions or macros
which read registers can now be used in expressions.

Change-Id: I7a463717051efd2f9dd36e8a84d357852fbf9215
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>