net: mscc: ocelot: make access to STAT_VIEW sleepable again

To support SPI-controlled switches in the future, access to
SYS_STAT_CFG_STAT_VIEW needs to be done outside of any spinlock
protected region, but it still needs to be serialized (by a mutex).

Split the ocelot->stats_lock spinlock into a mutex that serializes
indirect access to hardware registers (ocelot->stat_view_lock) and a
spinlock that serializes access to the u64 ocelot->stats array.

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