arm: mvebu: Update defconfig with Marvell RTC support

The RTC class driver is already part of the mvebu_defconfig but the
Marvell internal RTC not yet. Now that its support is added for mvebu
let's update the config file.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>