net: dsa: bcm_sf2: Configure IMP port for 2Gb/sec

With the implementation of the system reset controller we lost a setting
that is currently applied by the bootloader and which configures the IMP
port for 2Gb/sec, the default is 1Gb/sec. This is needed given the
number of ports and applications we expect to run so bring back that
setting.

Fixes: 01b0ac07589e ("net: dsa: bcm_sf2: Add support for optional reset controller line")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>