net: dsa: sja1105: skip CGU configuration if it's unnecessary

net: dsa: sja1105: skip CGU configuration if it's unnecessary

There are two distinct code paths which enter sja1105_clocking.c, one
through sja1105_clocking_setup() and the other through
sja1105_clocking_setup_port():

sja1105_static_config_reload      sja1105_setup
              |                         |
              |      +------------------+
              |      |
              v      v
   sja1105_clocking_setup               sja1105_adjust_port_config
                 |                                   |
                 v                                   |
      sja1105_clocking_setup_port <------------------+

As opposed to SJA1105, the SJA1110 does not need any configuration of
the Clock Generation Unit in order for xMII ports to work. Just RGMII
internal delays need to be configured, and that is done inside
sja1105_clocking_setup_port for the RGMII ports.

So this patch introduces the concept of a "reserved address", which the
CGU configuration functions from sja1105_clocking.c must check before
proceeding to do anything. The SJA1110 will have reserved addresses for
the CGU PLLs for MII/RMII/RGMII.

Additionally, make sja1105_clocking_setup() a function pointer so it can
be overridden by the SJA1110. Even though nothing port-related needs to
be done in the CGU, there are some operations such as disabling the
watchdog clock which are unique to the SJA1110.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>