ARM: dts: NSP: Add Switch Register Access Block node

Add the Switch Register Access Block node, this peripheral is identical
to the BCM5301x Northstar SoC, but we utilize the SoC-wide
"brcm,nsp-srab" compatible string to illustrate the integration
difference here.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>