ARM: ixp4xx: avoid use of PCIBIOS_MIN_MEM in io.h

When using CONFIG_IXP4XX_INDIRECT_PCI, we run into a recursive
header file dependency between mach/io.h and asm/pci.h, resulting
in a build failure:

mach-ixp4xx/include/mach/io.h: In function 'is_pci_memory':
mach-ixp4xx/include/mach/io.h:53:18: error: 'PCIBIOS_MIN_MEM' undeclared (first use in this function)
  return (addr >= PCIBIOS_MIN_MEM) && (addr <= 0x4FFFFFFF);
                  ^
mach-ixp4xx/include/mach/io.h:53:18: note: each undeclared identifier is reported only once for each function it appears in

We can work around this by referencing the pcibios_min_mem variable
directly through an extern declaration, rather than using the macro.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>