phy/micrel: Add missing header to micrel_phy.h

The license header was missing in micrel_phy.h . This patch adds
one.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: David J. Choi <david.choi@micrel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>