arm64: dts: apq8096-db820c: disable uart0 by default

Access to UART0 is disabled by bootloaders. By leaving it enabled by
default would reboot the board.
Disable this for now, this would alteast give a board which boots.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>