nios2: Extend !__ASSEMBLY__ section in asm/page.h

Some C-only definition from nios2 asm/page.h are exposed to assembly code.
Extend the !__ASSEMBLY__ section to prevent this.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>