net: dsa: rework switch parsing

When parsing a switch, we have to identify to which tree it belongs and
parse its ports. Provide two functions to separate the OF and platform
data specific paths.

Also use the of_property_read_variable_u32_array function to parse the
OF member array instead of calling of_property_read_u32_index twice.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>