ARC: Remove redundant PCI_IOBASE declaration

ARC's asm/io.h includes the asm-generic/io.h which already defines the
PCI_IOBASE variable in exactly the same way, so it can be dropped from
the architecture specific header.
Signed-off-by: Thierry Reding <treding@nvidia.com>

arch/arc/include/asm/io.h

@@ -13,8 +13,6 @@
 #include <asm/byteorder.h>
 #include <asm/page.h>
 
-#define PCI_IOBASE ((void __iomem *)0)
-
 extern void __iomem *ioremap(unsigned long physaddr, unsigned long size);
 extern void __iomem *ioremap_prot(phys_addr_t offset, unsigned long size,
 				  unsigned long flags);