ARM: imx_v6_v7_defconfig: add CONFIG_RTC_DRV_M41T80

The rv4168 RTC is used by the following platforms:
- Nitrogen6_MAX (both Quad and Quad Plus versions)
- Nitrogen7

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>