drivers/rtc/rtc-mpc5121.c: add support for RTC on MPC5200

MPC5200B contains a limited version of RTC from MPC5121.  Add support for
the RTC on that CPU.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>