net: phy: Allow BCM5481x PHYs to setup internal TX/RX clock delay

This patch allows users to enable/disable internal TX and/or RX
clock delay for BCM5481x series PHYs so as to satisfy RGMII timing
specifications.

On a particular platform, whether TX and/or RX clock delay is required
depends on how PHY connected to the MAC IP. This requirement can be
specified through "phy-mode" property in the platform device tree.

Signed-off-by: Abhishek Shah <abhishek.shah@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>