net: dsa: mv88e6xxx: read cycle counter period from hardware

net: dsa: mv88e6xxx: read cycle counter period from hardware

[ Upstream commit 7e3c18097a709e9b958e721066e5fe76e563739b ]

Instead of relying on a fixed mapping of hardware family to cycle
counter frequency, pull this information from the
MV88E6XXX_TAI_CLOCK_PERIOD register.

This lets us support switches whose cycle counter frequencies depend on
board design.

Fixes: de776d0d316f ("net: dsa: mv88e6xxx: add support for mv88e6393x family")
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shenghao Yang <me@shenghaoyang.info>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>