[PPC] Fix compile error for e500 core based processors

We get the following compiler error:

  CC      arch/ppc/kernel/ppc_ksyms.o
arch/ppc/kernel/ppc_ksyms.c:275: error: '__mtdcr' undeclared here (not in a function)
arch/ppc/kernel/ppc_ksyms.c:275: warning: type defaults to 'int' in declaration of '__mtdcr'
arch/ppc/kernel/ppc_ksyms.c:276: error: '__mfdcr' undeclared here (not in a function)
arch/ppc/kernel/ppc_ksyms.c:276: warning: type defaults to 'int' in declaration of '__mfdcr'
make[1]: *** [arch/ppc/kernel/ppc_ksyms.o] Error 1

This is due to the EXPORT_SYMBOL for __mtdcr/__mfdcr not having the proper CONFIG protection

Signed-off-by: Rojhalat Ibrahim <imr@rtschenk.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>