nrf52: dts: Force user to explicitly set UART0 compatible

Since the UART0 @ 0x40002000 can either be UART or UARTE the user of the
soc.dtsi needs to select either compatible = "nordic,nrf-uarte" or
"nordic,nrf-uart"

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>