net: mv643xx_eth: add shared clk and cleanup existing clk handling

This patch adds an optional shared block clock to avoid lockups on
clock gated controllers. Besides the new clock, clock handling for
existing clocks is cleaned up and moved to devm_clk_get. Device
tree binding documentation is updated for the new clocks property.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>