of: make OF_EARLY_FLATTREE depend on HAS_IOMEM

On s390 systems (aka mainframes), it has classic channel devices for
networking and permanent storage that are currently even more common
than PCI devices. Hence it could have a fully functional s390 kernel
with CONFIG_PCI=n, then the relevant iomem mapping functions
[including ioremap(), devm_ioremap(), etc.] are not available.

In LKP error report at below on s390:
------
ld: kernel/dma/coherent.o: in function `dma_init_coherent_memory':
coherent.c:(.text+0x102): undefined reference to `memremap'
ld: coherent.c:(.text+0x226): undefined reference to `memunmap'
ld: kernel/dma/coherent.o: in function `dma_declare_coherent_memory':
coherent.c:(.text+0x8b8): undefined reference to `memunmap'
ld: kernel/dma/coherent.o: in function `dma_release_coherent_memory':
coherent.c:(.text+0x9aa): undefined reference to `memunmap'
------

In the config file, several Kconfig options are:
------
'# CONFIG_PCI is not set'
CONFIG_OF_EARLY_FLATTREE=y
CONFIG_DMA_DECLARE_COHERENT=y
------

So, enabling OF_EARLY_FLATTREE will select DMA_DECLARE_COHERENT
and cause above building errors even though they are not needed
because CONFIG_PCI is disabled.

Here let OF_EARLY_FLATTREE depend on HAS_IOMEM so that it won't
be built to cause compiling error if PCI is unset.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306211329.ticOJCSv-lkp@intel.com/
Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20230707135852.24292-9-bhe@redhat.com
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/Kconfig

@@ -55,7 +55,7 @@ config OF_FLATTREE
 
 config OF_EARLY_FLATTREE
 	bool
-	select DMA_DECLARE_COHERENT if HAS_DMA
+	select DMA_DECLARE_COHERENT if HAS_DMA && HAS_IOMEM
 	select OF_FLATTREE
 
 config OF_PROMTREE