net: dsa: b53: Introduce b53_adjust_531x5_rgmii()

Takes care of doing the 531x5 switch series specific RGMII programming
and is called from b53_adjust_link() to allow the future removal of
b53_adjust_link().

Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/20240423183339.1368511-3-florian.fainelli@broadcom.com


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