net: phy: Add Broadcom phy library for common interfaces

This patch adds the Broadcom phy library to consolidate common
interfaces shared by Broadcom phy's.

Moved the common interfaces to the 'bcm-phy-lib.c' and updated
the Broadcom PHY drivers to use the new APIs.

Signed-off-by: Arun Parameswaran <arunp@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>