Documentation: networking: phy.txt: Update text for indirect MMD access

Update the PHY library documentation to describe how a specific PHY
driver can use the PAL MMD register access routines or override those
routines with it's own in the event the PHY does not support the IEEE
standard for reading and writing MMD phy registers.

Signed-off-by: Vince Bridgers <vbridgers2013@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>