net: phy: Add helper for getting MAC termination resistance

Add helper which returns the MAC termination resistance value. Modifying
the resistance to an appropriate value can reduce signal reflections and
therefore improve signal quality.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Dimitri Fedrau <dimitri.fedrau@liebherr.com>
Link: https://patch.msgid.link/20250416-dp83822-mac-impedance-v3-3-028ac426cddb@liebherr.com


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