mlxsw: spectrum: split base on local_ports_in_{1x, 2x} resources

When splitting a port, different local ports need to be mapped on different
systems. For example:

SN3700 (local_ports_in_2x=2):
  * Without split:
      front panel 1   --> local port 1
      front panel 2   --> local port 5
  * Split to 2:
      front panel 1s0 --> local port 1
      front panel 1s1 --> local port 3
      front panel 2   --> local port 5

SN3800 (local_ports_in_2x=1):
  * Without split:
      front panel 1 --> local port 1
      front panel 2 --> local port 3
  * Split to 2:
      front panel 1s0 --> local port 1
      front panel 1s1 --> local port 2
      front panel 2   --> local port 3

The local_ports_in_{1x, 2x} resources provide the offsets from the base
local ports according to which the new local ports can be calculated.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>