net: phy: add paged PHY register accessors

net: phy: add paged PHY register accessors

Synchronize paged PHY helpers with Linux v6.17.

Add support for PHY devices that use paged register access by
implementing the following functions:
- phy_save_page(): Save current page number
- phy_select_page(): Switch to a specific page and return previous page
- phy_restore_page(): Restore previously saved page

Also adds read_page and write_page callbacks to the phy_driver
structure to enable driver-specific page handling.

These helpers allow safe access to paged PHY registers by ensuring
proper page selection and restoration,
even in error conditions, which will be used by the Airoha PHY driver.

Signed-off-by: Lucien.Jheng <lucienzx159@gmail.com>