dt-bindings: net: sff,sfp: update binding

Add a minimum and default for the maximum-power-milliwatt option;
module power levels were originally up to 1W, so this is the default
and the minimum power level we can have for a functional SFP cage.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>