block: pmem: Add dependency on HAS_IOMEM

Not all architectures have io memory.

Fixes:
drivers/block/pmem.c: In function ‘pmem_alloc’:
drivers/block/pmem.c:146:2: error: implicit declaration of function ‘ioremap_nocache’ [-Werror=implicit-function-declaration]
  pmem->virt_addr = ioremap_nocache(pmem->phys_addr, pmem->size);
  ^
drivers/block/pmem.c:146:18: warning: assignment makes pointer from integer without a cast [enabled by default]
  pmem->virt_addr = ioremap_nocache(pmem->phys_addr, pmem->size);
                  ^
drivers/block/pmem.c:182:2: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration]
  iounmap(pmem->virt_addr);
  ^

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>