net: phy: dp83826: support configuring RMII master/slave operation mode

The TI DP83826 PHY can operate between two RMII modes:
	- master mode (PHY operates from a 25MHz clock reference)
        - slave mode (PHY operates from a 50MHz clock reference)

By default, the operation mode is configured by hardware straps.

Add support to configure the operation mode from within the driver.

Signed-off-by: Jérémie Dautheribes <jeremie.dautheribes@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>