drivers: net: davinci_mdio: implement pm runtime auto mode

Davinci MDIO is always used as slave device which services
read/write requests from MDIO/PHY core. It doesn't use IRQ also.

As result, It's possible to relax PM runtime constraints for Davinci
MDIO and enable it on demand, instead of powering it during probe
and powering off during removal.

Hence, implement PM runtime autosuspend for Davinci MDIO, but keep it
disabled by default, because Davinci MDIO is integrated in big set of
TI devices and not all of them expected to work corectly with RPM
 autosuspend enabled:
- expected to work on SoCs where MDIO is part of TI CPSW
(cpsw.c DRA7/am57x, am437x, am335x, dm814x)
- not verified on Keystone 2 and other SoCs where MDIO is used with TI EMAC IP
(davinci_emac.c:  dm6467-emac, am3517-emac, dm816-emac).

Davinci MDIO RPM autosuspend can be enabled through sysfs:
 echo 100 > /sys/devices/../48484000.ethernet/48485000.mdio/power/autosuspend_delay_ms

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>