Because dfl.c uses the 'devm_ioremap', 'devm_iounmap',
'devm_ioremap_resource', and 'devm_platform_ioremap_resource'
functions, it should depend on HAS_IOMEM.

This fixes make allyesconfig under UML (ARCH=um), which doesn't provide
HAS_IOMEM.

[mdf@kernel.org: Removed "drivers: " in commit message]
Fixes: 89eb35e8 ("fpga: dfl: map feature mmio resources in their own feature drivers")
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Link: https://lore.kernel.org/r/20201122001549.107023-2-mdf@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>