net: macb: Support clock management for tsu_clk

TSU clock needs to be enabled/disabled as per support in devicetree
and it should also be controlled during suspend/resume (WOL has no
dependency on this clock).

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>