mlxsw: item: Add MLXSW_ITEM_BUF_INDEXED helper

Add missing item helper which allows to access char bufs on multiple
offsets. This is needed by SFD and SFN register definitions.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>