net: ll_temac: Allow configuration of IRQ coalescing

This allows custom setup of IRQ coalescing for platforms using legacy
platform_device. The irq timeout and count parameters can be used for
tuning cpu load vs. latency.

I have maintained the 0x00000400 bit in TX_CHNL_CTRL.  It is specified as
unused in the documentation I have available.  It does not make any
difference in the hardware I have available, so it is left in to not risk
breaking other platforms where it might be used.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>