vscphy: Add support for PHY interrupts

They're allocated using standard newbus API,
which means that we rely on miibus to handle the allocation.
Add VSC8504 to the list of supported PHYs, as it is similar enough
to the VSC8501 that is already supported by this driver.

Obtained from: Semihalf
Sponsored by: Alstom Group
Differential revision: https://reviews.freebsd.org/D32816