ARM: dts: NSP: Add XMC board support

The BCM958525XMC board is a smaller form factor typically used as
controller boards for switches.  This smaller board has less devices
pinned out, so only a few need be populated in the device tree.

Signed-off-by: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>