rtc: Add MPC5121 Real time clock driver

Add support for MPC5121 real time clock module.

Signed-off-by: John Rigby <jcrigby@gmail.com>
Signed-off-by: Piotr Ziecik <kosmo@semihalf.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>