net: dsa: bcm_sf2: Prevent GPHY shutdown for kexec'd kernels

For a kernel that is being kexec'd we re-enable the integrated GPHY in
order for the subsequent MDIO bus scan to succeed and properly bind to
the bcm7xxx PHY driver. If we did not do that, the GPHY would be shut
down by the time the MDIO driver is probing the bus, and it would fail
to read the correct PHY OUI and therefore bind to an appropriate PHY
driver. Later on, this would cause DSA not to be able to successfully
attach to the PHY, and the interface would not be created at all.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>