powerpc/83xx: add DS1374 RTC support for the MPC837xE-MDS boards

The RTC is sitting on the I2C1 bus at address 0x68. RTC interrupt signal
is connected to the IPIC's EXT3 interrupt line.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>