asm-generic: ioremap_uc should behave the same with and without MMU

Whatever reason there is for the existence of ioremap_uc, and the fact
that it returns NULL by default on architectures with an MMU applies
equally to nommu architectures, so don't provide different defaults.

In practice the difference is meaningless as the only portable driver
that uses ioremap_uc is atyfb which probably doesn't show up on nommu
devices.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Palmer Dabbelt <palmer@dabbelt.com>