net: dsa: mt7530: Add MT7621 TRGMII mode support

This patch add support TRGMII mode for MT7621 internal MT7530 switch.
MT7621 TRGMII has only one fix speed mode of 1200MBit.

Also adding support for mt7530 25MHz and 40MHz crystal clocksource.
Values are based on Banana Pi R2 bsp [1].

Don't change MT7623 registers on a MT7621 device.

[1] https://github.com/BPI-SINOVOIP/BPI-R2-bsp/blob/master/linux-mt/drivers/net/ethernet/mediatek/gsw_mt7623.c#L769

Signed-off-by: René van Dorst <opensource@vdorst.com>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: David S. Miller <davem@davemloft.net>