ARM: mvebu: update defconfig with I2C and RTC support

Now that we have support for the I2C busses on Armada 370/XP, and
support for the RTC on the OpenBlocks AX3-4 platform, include the
necessary options in mvebu_defconfig.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>