net: phy: Document phydev::dev_flags bits allocation

Document the phydev::dev_flags bit allocation to allow bits 15:0 to
define PHY driver specific behavior, bits 23:16 to be reserved for now,
and bits 31:24 to hold generic PHY driver flags.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/20210526184617.3105012-1-f.fainelli@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>