net: lantiq: lantiq_xrx200: Move clock prepare to probe function

The switch and the MAC are in one IP core and they use the same clock
signal from the clock generation unit.
Currently the clock architecture in the lantiq SoC code does not allow
to easily share the same clocks, this has to be fixed by switching to
the common clock framework.
As a workaround the clock of the switch and MAC should be activated when
the MAC gets probed and only disabled when the MAC gets removed. This
way it is ensured that the clock is always enabled when the switch or
MAC is used. The switch can not be used without the MAC.

This fixes a data bus error when rebooting the system and deactivating
the switch and mac and later accessing some registers in the cleanup
while the clocks are disabled.

Fixes: fe1a5642 ("net: lantiq: Add Lantiq / Intel VRX200 Ethernet driver")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>