powerpc: Drop probecpu() in favour of arch_cpu_init()

To avoid an unnecessary arch-specific call in board_init_f(), rename this
function.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/Arcturus/ucp1020/spl.c b/board/Arcturus/ucp1020/spl.c
index 9c19c87..3a775be 100644
--- a/board/Arcturus/ucp1020/spl.c
+++ b/board/Arcturus/ucp1020/spl.c
@@ -83,7 +83,7 @@
 	bd->bi_memstart = CONFIG_SYS_INIT_L2_ADDR;
 	bd->bi_memsize = CONFIG_SYS_L2_SIZE;
 
-	probecpu();
+	arch_cpu_init();
 	get_clocks();
 	mem_malloc_init(CONFIG_SPL_RELOC_MALLOC_ADDR,
 			CONFIG_SPL_RELOC_MALLOC_SIZE);