ARM: dts: NSP: Add GPIO reboot method to bcm958625xmc DTS file

Add the ability to reboot the bcm958625xmc board via GPIO.
Unfortunately, not all of the NSP based boards use the same GPIO pin
and one doesn't have the ability to reboot via GPIO at all.  So, this
will need to be specified per DTS file.

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