net: dsa: bcm_sf2: Prepare for different register layouts

In preparation for supporting a new device with a slightly different
register layout, affecting the SWITCH_REG and SWITCH_CORE address
spaces, perform a few preparatory steps:

- allow matching the compatible string against a data description
- convert the SWITCH_REG register accesses into an indirection table
- prepare for supporting a SWITCH_CORE register alignment requirement

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>