net: dsa: mv88e6xxx: global2: Expose ATU stats register

[ Upstream commit 6239a386e784aed13c3ead54c3992ebcb0512d5f ]

Add helpers to set/get the ATU statistics register.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 528876d867a2 ("net: dsa: mv88e6xxx: Fix out-of-bound access")
Signed-off-by: Sasha Levin <sashal@kernel.org>