nios2: define virtual address space for modules

nios2 uses kmalloc() to implement module_alloc() because CALL26/PCREL26
cannot reach all of vmalloc address space.

Define module space as 32MiB below the kernel base and switch nios2 to
use vmalloc for module allocations.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Dinh Nguyen <dinguyen@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>