phy: adin: add readext and writeext support for mdio cmd

The adin phy has extended registers that can be accessed using
adin_ext_read and adin_ext_write. These registers can be read directly
using the mdio command using readext and writext. For example:

     => mdio rx ethernet@428a0000 0xff23
     Reading from bus ethernet@428a0000
     PHY at address 0:
     65315 - 0xe01

Signed-off-by: Nate Drude <nate.d@variscite.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>