net: cadence: macb: Add support for Raspberry Pi RP1 ethernet controller

The RP1 chip has the Cadence GEM block, but wants the tx_clock
to always run at 125MHz, in the same way as sama7g5.
Add the relevant configuration.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Stanimir Varbanov <svarbanov@suse.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Link: https://patch.msgid.link/20250916081059.3992108-1-svarbanov@suse.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>