net: phy: micrel: make *-skew-ps check more lenient

It seems reasonable to fine-tune only some of the skew values when using
one of the rgmii-*id PHY modes, and even when all skew values are
specified, using the correct ID PHY mode makes sense for documentation
purposes. Such a configuration also appears in the binding docs in
Documentation/devicetree/bindings/net/micrel-ksz90x1.txt, so the driver
should not warn about it.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Link: https://lore.kernel.org/r/20211012103402.21438-1-matthias.schiffer@ew.tq-group.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>