platform: mellanox: mlx-platform: Add mux selection register to regmap

Extend writeable, readable, volatile registers of the 'regmap' object
with for I2C mux selector registers.

The motivation is to pass this object extended with selector registers
to I2C mux driver working over ‘regmap’.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Michael Shych <michaelsh@nvidia.com>
Link: https://lore.kernel.org/r/20230208063331.15560-12-vadimp@nvidia.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>