net: dsa: mv88e6xxx: add accessors for PTP/TAI registers

This patch implements support for accessing the Precision Time Protocol
and Time Application Interface registers via the AVB register interface
in the Global 2 register.

The register interface differs slightly between different models; older
models use a 3-bit operations field, while newer models use a 2-bit
field. The operations values and the special "global port" values are
different between the two. This is a similar split to the differences
in the "Ingress Rate" register between models, so, like in that case,
we call the two variants "6352" and "6390" and create an ops structure
to abstract between the two.

checkpatch fixups by Andrew Lunn

Signed-off-by: Brandon Streiff <brandon.streiff@ni.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>