powerpc/83xx: Add DS1339 RTC support for MPC8349E-mITX boards .dts

The RTC is sitting on the I2C2 bus at address 0x68. RTC interrupt signal
is connected to the IPIC's EXT2 interrupt line, the line is shared with
Vitesse 8201 Ethernet PHY.

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