arm64: dts: Add pinctrl DT nodes for Stingray SOC

This patch adds pinctrl and pinmux related DT nodes for
Stingray SOC.

For manageability, pinctrl and pinmum DT nodes are added
as separate DTSi file and included in main DTSi file.

Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>