net: dsa: mv88e6xxx: introduce helpers for handling chip->reg_lock

This is a no-op that simply moves all locking and unlocking of
->reg_lock into trivial helpers. I did that to be able to easily add
some ad hoc instrumentation to those helpers to get some information
on contention and hold times of the mutex. Perhaps others want to do
something similar at some point, so this frees them from doing the
'sed -i' yoga, and have a much smaller 'git diff' while fiddling.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>