mpl: printing current stdio devices cleanup

Currently the mpl boards duplicate the code to print the current
devices from common/console.c; use stdio_print_current_devices()
instead

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

Edited commit message.

Signed-off-by: Wolfgang Denk <wd@denx.de>