net: dsa: mt7530: Constify struct regmap_config

'struct regmap_config' are not modified in these drivers. They be
statically defined instead of allocated and populated at run-time.

The main benefits are:
  - it saves some memory at runtime
  - the structures can be declared as 'const', which is always better for
    structures that hold some function pointers
  - the code is less verbose

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>