arm64: dts: stingray: move common board components to stingray-board-base

Move common board components from base bcm958742 dtsi file to new
stingray-board-base dtsi file so they can be shared between many stingray
boards following common design.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>