net: gemini: allow any RGMII interface mode

The four RGMII interface modes take care of the required RGMII delay
configuration at the PHY and should not be limited by the network MAC
driver. Sadly, gemini was only permitting RGMII mode with no delays,
which would require the required delay to be inserted via PCB tracking
or by the MAC.

However, there are designs that require the PHY to add the delay, which
is impossible without Gemini permitting the other three PHY interface
modes. Fix the driver to allow these.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Link: https://lore.kernel.org/r/E1n4mpT-002PLd-Ha@rmk-PC.armlinux.org.uk


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