[BUG] Small issues on iMXRT found by Claude #20692

Description / Steps to reproduce the issue

Two smaller upstream issues I noticed but left alone:

• PHY address scan: imxrt_determine_phy() never stores the PHY address it finds,
  so address ranges in BOARD_ETH0_PHY_LIST don't work. The EC300 isn't affected
  because it uses a single address.
• YT8512 config name: imxrt_phyintenable() checks CONFIG_ETH0_YT8512, but the
  Kconfig option is CONFIG_ETH0_PHY_YT8512.

On which OS does this issue occur?

[OS: Linux]

What is the version of your OS?

Ubuntu

NuttX Version

mainline

Issue Architecture

[Arch: arm]

Issue Area

[Area: Networking]

Host information

No response

Verification

• I have verified before submitting the report.