eth/eth_mcux: Provide start and stop operations on the PHY driver.

Refactor the PHY state machine and add support for explicit start and
stop.

The stop implementation remains partial, the state machine will enter
a disabled state but will not actual attempt to power down the PHY.
This is deliberate, while implementing this it has become apparent
that issuing a PHY power down command is an effective way of bricking
frdm-k64f boards, hence explicit power down deliberately disabled
until the issue is properly understood.

Change-Id: I846a51b0ac48feed35d260cf20b50f4f1ac59298
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>