ARM: highbank: add ifdef guards to core.h

Fixes build error due to multiple includes:

In file included from arch/arm/mach-highbank/sysregs.h:23:0,
from arch/arm/mach-highbank/system.c:20:
arch/arm/mach-highbank/core.h:8:91: error: redefinition of 'highbank_lluart_map_io'
In file included from arch/arm/mach-highbank/system.c:19:0:
arch/arm/mach-highbank/core.h:8:91: note: previous definition of 'highbank_lluart_map_io' was here
Signed-off-by: Olof Johansson <olof@lixom.net>

arch/arm/mach-highbank/core.h

+#ifndef __HIGHBANK_CORE_H
+#define __HIGHBANK_CORE_H
+
 extern void highbank_set_cpu_jump(int cpu, void *jump_addr);
 extern void highbank_clocks_init(void);
 extern void highbank_restart(char, const char *);
@@ -18,3 +21,5 @@ extern void highbank_smc1(int fn, int arg);
 extern void highbank_cpu_die(unsigned int cpu);
 
 extern struct smp_operations highbank_smp_ops;
+
+#endif