net: phy: phy-c45: add OATC14 10BASE-T1S PHY cable diagnostic support

net: phy: phy-c45: add OATC14 10BASE-T1S PHY cable diagnostic support

Add support for Open Alliance TC14 (OATC14) 10BASE-T1S PHYs cable
diagnostic feature.

This patch implements:
- genphy_c45_oatc14_cable_test_start() to initiate a cable test
- genphy_c45_oatc14_cable_test_get_status() to retrieve test results
- Helper function to map PHY cable test status to ethtool result codes
- Function declarations and exports for use by PHY drivers

This enables ethtool to report ok, open, short, and undetectable cable
conditions on OATC14 10Base-T1S PHYs.

Open Alliance TC14 10BASE-T1S Advanced Diagnostic PHY Features
Specification ref:
https://opensig.org/wp-content/uploads/2025/06/OPEN_Alliance_10BASE-T1S_Advanced_PHY_features_for-automotive_Ethernet_V2.1b.pdf

Signed-off-by: Parthiban Veerasooran <parthiban.veerasooran@microchip.com>
Link: https://patch.msgid.link/20251105051213.50443-2-parthiban.veerasooran@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>