arm64: dts: lx2160a-cex7: increase at8035 PHY gigabit Tw parameter

Increase the SmartEEE Tw parameter for Atheros PHYs to stop gigabit
links from sporadically dropping. Testing on this platform shows that
a value of 24 results in a stable link, whereas 23 or below has the
occasional drop.

Tested with a Netgear GS116 unmanaged switch link partner with Cat 5e
cabling.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>