ARM: fix ioremap/iounmap for !CONFIG_MMU

With commit 4fe7ef3a (ARM: provide runtime hook for ioremap/iounmap),
compiles with !CONFIG_MMU were broken. Rename nommu __iounmap to
__arm_iounmap and add arch_ioremap_caller and arch_iounmap. Its
not expected that these need to be overriden for !CONFIG_MMU, so setting
the function ptrs has no effect in this case.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Russell King <linux@arm.linux.org.uk>

arch/arm/mm/nommu.c

@@ -86,13 +86,17 @@ void __iomem *__arm_ioremap(unsigned long phys_addr, size_t size,
 }
 EXPORT_SYMBOL(__arm_ioremap);
 
+void __iomem * (*arch_ioremap_caller)(unsigned long, size_t, unsigned int, void *);
+
 void __iomem *__arm_ioremap_caller(unsigned long phys_addr, size_t size,
 				   unsigned int mtype, void *caller)
 {
 	return __arm_ioremap(phys_addr, size, mtype);
 }
 
-void __iounmap(volatile void __iomem *addr)
+void (*arch_iounmap)(volatile void __iomem *);
+
+void __arm_iounmap(volatile void __iomem *addr)
 {
 }
-EXPORT_SYMBOL(__iounmap);
+EXPORT_SYMBOL(__arm_iounmap);