dts: nrf9160: Add rtc0 node and replace nvmc node with alias

Add missing node for instance 0 of the RTC peripheral and use an alias
to the flash-controller node instead of the nvmc node definition (there
is no need to have a separate node just to get the NVMC base address).

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>