ARM: nommu: provide dummy cpu_switch_mm implementation

cpu_switch_mm is a logical nop on nommu systems, so define it as such
when !CONFIG_MMU.
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm/include/asm/proc-fns.h

@@ -137,6 +137,10 @@ extern void cpu_resume(void);
 	})
 #endif
 
+#else	/*!CONFIG_MMU */
+
+#define cpu_switch_mm(pgd,mm)	{ }
+
 #endif
 
 #endif /* __ASSEMBLY__ */