net: dsa: b53: mmap: Implement bcm63xx ephy power control

Implement the phy enable/disable calls for b53 mmap, and
set the power down registers in the ephy control register
appropriately.

Signed-off-by: Kyle Hendry <kylehendrydev@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20250724035300.20497-8-kylehendrydev@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>