net: axienet: add coalesce timer ethtool configuration

[ Upstream commit 0b79b8dc ]

Add the ability to configure the RX/TX coalesce timer with ethtool.
Change default setting to scale with the clock rate rather than being a
fixed number of clock cycles.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 5a6caa2c ("net: xilinx: axienet: Fix packet counting")
Signed-off-by: Sasha Levin <sashal@kernel.org>