phy: atheros: add support for RGMII_ID, RGMII_TXID and RGMII_RXID

This adds support for internal delay on RX and TX on RGMII interface for the
AR8035 phy.

This is basically the same Linux driver do. Tested on a Zynq Zturn board (for
which u-boot support in is my tree; first patch waiting ML approval)

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>