eth/mcux: Add basic PHY support.

Add basic PHY management sufficient to detect link up, link down and
auto negotiated link speed / duplex.  The PHY driver is implemented as
a state machine that executed in the system work queue.  The
implementation is non blocking, using the MII interrupt to capture the
completion of read and write events.

This PHY management should be fairly generic. In the future, it may be
beneficial to pull this code out as a standalone PHY driver for use
with other ethernet drivers.

JIRA: ZEP-1674

Change-Id: I3dcb5c14982ef4b40591fcf10d84840b8a2558e5
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>