net: lan966x: use the FDMA library for allocation of tx buffers

Use the two functions: fdma_alloc_phys() and fdma_dcb_init() for rx
buffer allocation and use the new buffers throughout.

In order to replace the old buffers with the new ones, we have to do the
following refactoring:

    - use fdma_alloc_phys() and fdma_dcb_init()

    - replace the variables: tx->dma, tx->dcbs and tx->curr_entry
      with the equivalents from the FDMA struct.

    - add lan966x_fdma_tx_dataptr_cb callback for obtaining the dataptr.

    - Initialize FDMA struct values.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>