net: dsa: microchip: add an enum for regmap widths

It is not immediately obvious that this driver allocates, via the
KSZ_REGMAP_TABLE() macro, 3 regmaps for register access: dev->regmap[0]
for 8-bit access, dev->regmap[1] for 16-bit and dev->regmap[2] for
32-bit access.

In future changes that add support for reg_fields, each field will have
to specify through which of the 3 regmaps it's going to go. Add an enum
now, to denote one of the 3 register access widths, and make the code go
through some wrapper functions for easier review and further
modification.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>