riscv: only select GENERIC_IOREMAP if MMU support is enabled

nommu ioremap is an inline stub in asm-generic/io.h.

Link: https://lkml.kernel.org/r/20210825072036.GA29161@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>