MIPS: Fix multiple definitions of UNCAC_BASE.

Fix build error below:

arch/mips/include/asm/mach-generic/spaces.h:29:0: warning:
"UNCAC_BASE" redefined [enabled by default]
In file included from arch/mips/include/asm/addrspace.h:13:0,
                 from arch/mips/include/asm/barrier.h:11,
                 from arch/mips/include/asm/bitops.h:18,
                 from include/linux/bitops.h:22,
                 from include/linux/kernel.h:10,
                 from include/asm-generic/bug.h:13,
                 from arch/mips/include/asm/bug.h:41,
                 from include/linux/bug.h:4,
                 from include/linux/page-flags.h:9,
                 from kernel/bounds.c:9:
arch/mips/include/asm/mach-ar7/spaces.h:20:0: note: this is the
location of the previous definition
Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5583/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/mach-generic/spaces.h

@@ -25,8 +25,12 @@
 #else
 #define CAC_BASE		_AC(0x80000000, UL)
 #endif
+#ifndef IO_BASE
 #define IO_BASE			_AC(0xa0000000, UL)
+#endif
+#ifndef UNCAC_BASE
 #define UNCAC_BASE		_AC(0xa0000000, UL)
+#endif
 
 #ifndef MAP_BASE
 #ifdef CONFIG_KVM_GUEST