emac: Fix clock control for 405EX and 405EXr chips

The EMAC variant in the 405EX and 405EXr chips needs the "440EP" type clock
control workaround to avoid lockups of the Rx side during reset.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Tested-by: Felix Radensky <felix@embedded-sol.com>
Signed-off-by: David S. Miller <davem@davemloft.net>