When CONFIG_PCI option is not set, the variables
pci_dram_offset, isa_io_base and isa_mem_base are not defined.

Currently, the test is handled in each platform header. This
patch moves the test in io.h once and for all.
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>