nios2: move include asm-generic/io.h to end of file

Move asm-generic/io.h to end of file to override functions like
phys_to_virt, virt_to_phys in asm-generic/io.h.
This is due to recent commit 9216efaf introduced new way to
override functions by checking for the existence of a macro with the
same of the function.

Signed-off-by: Ley Foon Tan <lftan@altera.com>