arm: mvebu: turris_mox: Convert comphy_update_map() to board_fix_fdt()

Code in board_fix_fdt() already detects connected MOX modules so there is
no need to have custom comphy_update_map() function for setting serdes
speeds.

This change sets phy-mode for MOX SFP module (when present) to sgmii.
Comphy driver then sets sgmii serdes speed for this module to 1.25G.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>