Add property ti,rmii-mode to support selecting the RMII operation mode
between:
	- master mode (PHY operates from a 25MHz clock reference)
	- slave mode (PHY operates from a 50MHz clock reference)

If not set, the operation mode is configured by hardware straps.
Signed-off-by: Jérémie Dautheribes <jeremie.dautheribes@bootlin.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>