ARM: dts: remove modem node in sda845 RB3 HW

RB3 is an open source HW for robotics using sda845.
Currently light-weighted modem firmware is not ready.
So pil_modem is diabled now until all other parts
are ready.

Change-Id: I4ee852ffbd89d8de617ccc6aa2ab4da2cafe644d
Signed-off-by: Tiequan Luo <tieqluo@codeaurora.org>