m68k: powerpc: Clean up do_mdm_init

This code seems unnecessarily complex. We really just need to check the
global_data. Now that is it all in one place, and not arch-specific, this
is pretty easy.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/common.h b/include/common.h
index 91745cf..3473ee5 100644
--- a/include/common.h
+++ b/include/common.h
@@ -299,6 +299,7 @@
 int mac_read_from_eeprom(void);
 extern u8 __dtb_dt_begin[];	/* embedded device tree blob */
 int set_cpu_clk_info(void);
+int mdm_init(void);
 #if defined(CONFIG_DISPLAY_CPUINFO)
 int print_cpuinfo(void);
 #else