ARM: imx6: Remove CONFIG_FEC_MXC_PHYADDR from DH header

Use phy address from device tree instead of CONFIG_FEC_MXC_PHYADDR from
board header. This is required, because the DH picoITX and DRC02 boards
require different settings than PDK2. The corresponding 'phy-handle'
device tree properties are already there.

I tested this change on picoITX and DRC02, but on PDK2 it is untested.

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Philip Oberfichtner <pro@denx.de>