ARM: dts: Cygnus: Add Broadcom iproc-static-adc DT node

This patch adds the DT node for Broadcom's iproc-static-adc
controller driver.

Signed-off-by: Raveendra Padasalagi <raveendra.padasalagi@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>