drivers: phy_ksz8081: Fix boot and link up

Two issues are being simultaneously address in this commit:

Issue 1:

Current issue is that zephyr boot / init is slow because it is blocked
by the phy ksz8081 init doing autonegotiation which can take on the
order of a few seconds.

Fix by now doing autonegotiation in the monitor handler instead of in
the cfg link call. The cfg link call will now only set the ANAR register
and a software flag to let the monitor handler know whether or not to
redo the autonegotiation sequence.

Issue 2:

The ksz8081 phy currently does not link up ever on cold boots on the NXP
RT platforms due to regression on mainline. My understanding of why is
not clear, but I found that re-setting the RMII override bit by the time
the monitor work handler runs with the change to fix the first issue,
makes the link come up.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>