libethdrivers,tx2: Fit TX,RX tail descriptors

The tail descriptors indicate the last descriptor that is available to
the DMA controller for the Ethernet module. While it is possible if we
set the tail descriptor to be the last descriptor in the allocated ring,
this could cause slowdowns when the DMA controller looks at a descriptor
in the ring that is not ready to be transmitted/received into.

This commit changes it so that the tail descriptors are set when we have
buffers ready and so also that it matches the last descriptor that is
available to be acted on.

libethdrivers,tx2: Stop spurious IRQs in TX

Disabling interrupts in the TX path prevents extra interrupts from
firing while handing the interrupts.

libethdrivers,tx2: Refactor RX IRQ handling code

This commits refactors the RX path IRQ handling code so the process of
disabling RX interrupts is outside of the IRQ status checking code. It
makes more sense to disable interrupts in the IRQ handling function and
not in the IRQ status checking code. Also rename one of fields in the
DMA registers struct so that it matches the actual functionality.

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.

tx2-ethernet: port Synopsis driver

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