arm64: dts: Initial DTS files for Broadcom Stingray SOC

The Broadcom Stingray SoC is a new member in Broadcom iProc
SoC family.

This patch adds initial DTS files for Broadcom Stingray SoC
and two of its reference boards (bcm958742k and bcm958742t).

We have lot of reference boards and large number of devices
in Broadcom Stingray SoC so eventually we will have quite
a few DTS files for Stingray. To tackle, we have added a
separate directory for Stingray DTS files.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>