net: phy: adin: implement PHY subsystem software reset

The ADIN PHYs supports 4 types of reset:
1. The standard PHY reset via BMCR_RESET bit in MII_BMCR reg
2. Reset via GPIO
3. Reset via reg GeSftRst (0xff0c) & reload previous pin configs
4. Reset via reg GeSftRst (0xff0c) & request new pin configs

Resets 2, 3 & 4 are almost identical, with the exception that the crystal
oscillator is available during reset for 2.

This change implements subsystem software reset via the GeSftRst and
reloading the previous pin configuration (so reset number 3).
This will also reset the PHY core regs (similar to reset 1).

Since writing bit 1 to reg GeSftRst is self-clearing, the only thing that
can be done, is to write to that register, wait a specific amount of time
(10 milliseconds should be enough) and try to read back and check if there
are no errors on read. A busy-wait-read won't work well, and may sometimes
work or not work.

In case phylib is configured to also do a reset via GPIO, the ADIN PHY may
be reset twice when the PHY device registers, but that isn't a problem,
since it's being done on boot (or PHY device register).

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>