[POWERPC] Add restart support for mpc52xx based platforms

Add common helper routines: mpc52xx_map_wdt() and mpc52xx_restart().

Signed-off-by: Marian Balakowicz <m8@semihalf.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>