powerpc/xmon: Fix build failure for 8xx

With CONFIG_PPC_8xx and CONFIG_XMON set, kernel build fails with

  arch/powerpc/xmon/xmon.c:1379:12: error: 'find_free_data_bpt' defined
  but not used [-Werror=unused-function]

Fix it by enclosing find_free_data_bpt() inside #ifndef CONFIG_PPC_8xx.

Fixes: 30df74d6 ("powerpc/watchpoint/xmon: Support 2nd DAWR")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201130034406.288047-1-ravi.bangoria@linux.ibm.com

arch/powerpc/xmon/xmon.c

@@ -1383,6 +1383,7 @@ static long check_bp_loc(unsigned long addr)
 	return 1;
 }
 
+#ifndef CONFIG_PPC_8xx
 static int find_free_data_bpt(void)
 {
 	int i;
@@ -1394,6 +1395,7 @@ static int find_free_data_bpt(void)
 	printf("Couldn't find free breakpoint register\n");
 	return -1;
 }
+#endif
 
 static void print_data_bpts(void)
 {