libethdrivers,tx2: Return bits in eqos_handle_irq

Now that the TX interrupt is also working, there may be a situation
where both RX and TX interrupts to fire at the same causing the common
interrupt line to mask either one of the two. This commit updates the
interrupt handler so that it returns a bitfield of interrupts seen so
that it avoids this situation.

Also replace the magic numbers in the driver with constants and make a
minor aesthetics change to make it more readable.

libethdrivers,tx2: Change TX path to be IRQ based

This commit changes the TX path to be IRQ based instead of the previous
polling approach, that is, to get the driver to fire an interrupt upon
the transmission of all the packets instead of polling until all packets
have been sent.

tx2-ethernet: port Synopsis driver

Port/reverse engineer driver from linux/l4t/u-boot sources.