drivers: Update Atmel SAM family GMAC Ethernet driver

Networking stack has split one global DATA pool to RX and TX DATA pools
and also added net_buf pool support to each context. Update the driver
to support this new design. Since the GMAC TX descriptor list has a fixed
size but the number of TX DATA buffers is no longer limited updating the
TX descriptor list has to be guarded by a semaphore.

Tested on Atmel SMART SAM E70 Xplained board

Change-Id: I181e1cdd183e173b85d5d1711b6e78cd5165666d
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>