dt-bindings: net: dsa: make reset optional and add rgmii-mode to mt7531

dt-bindings: net: dsa: make reset optional and add rgmii-mode to mt7531

A board may have no independent reset-line, so reset cannot be used
inside switch driver.

E.g. on Bananapi-R2 Pro switch and gmac are connected to same reset-line.

Resets should be acquired only to 1 device/driver. This prevents reset to
be bound to switch-driver if reset is already used for gmac. If reset is
only used by switch driver it resets the switch *and* the gmac after the
mdio bus comes up resulting in mdio bus goes down. It takes some time
until all is up again, switch driver tries to read from mdio, will fail
and defer the probe. On next try the reset does the same again.

Make reset optional for such boards.

Allow port 5 as cpu-port and phy-mode rgmii for mt7531.

- MT7530 supports RGMII on port 5 and RGMII/TRGMII on port 6.
- MT7531 supports on port 5 RGMII and SGMII (dual-sgmii) and
  SGMII on port 6.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>