nios2: Use asm-generic/io.h

Convert the nios2 architecture to make use of the new asm-generic/io.h to
provide address mapping functions. As nios2 actually performs
non-identity mapping between physical & virtual addresses we can't
simply make use of the generic functions, with the exception of being
able to drop our no-op unmap_physmem() and definitions of unused map
flags.

Feedback from architecture maintainers is welcome.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Thomas Chou <thomas@wytron.com.tw>