arm64: defconfig: Enable MT6397 RTC

The MT6397 RTC is part of the Mediatek MT6397 PMIC, which is present in
the mt8173-elm base board.

The driver for the PMIC is already enabled through CONFIG_MFD_MT6397, so
enable just the driver for the RTC.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Link: https://lore.kernel.org/r/20211217162445.876034-1-nfraprado@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>