arm64: dts: Add PWM and SDHCI DT nodes for Stingray SOC

The Stingray SoC has two instances of SDHCI controller
and one instance of iProc PWM.

Let's enable above mentioned devices in Stingray DT.

Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Signed-off-by: Anup Patel <anup.patel@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>