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=...