[POWERPC] Document rgmii-rxid and rgmii-txid phy-connection-types

A h/w bug requires we program the PHY in RGMII mode for internal delay
on the receive or transmit side only; document the new property values.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>