mb/siemens/mc_ehl6: Enable auto impedance calibration on GbE 0

Use the default automatic RGMII Output Impedance Calibration on Marvell
PHY 88E1512 by using default coreboot driver parameters.

TEST=Check signal integrity on RGMII interface on mc_ehl6 mainboard.

Change-Id: I6b6d7aeb8ddc4ef4ed297f4147b87d3e81c8a37e
Signed-off-by: Uwe Poeche <uwe.poeche@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90088
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kilian Krause <kilian.krause@siemens.com>
Reviewed-by: Mario Scheithauer <mario.scheithauer@siemens.com>

src/mainboard/siemens/mc_ehl/variants/mc_ehl6/devicetree.cb

@@ -211,9 +211,6 @@ chip soc/intel/elkhartlake
 				# INTn is not routed to LED[2] pin
 				register "enable_int" = "false"
 				register "downshift_cnt" = "2"
-				register "force_mos" = "true"
-				register "pmos_val" = "0xF"
-				register "nmos_val" = "0xA"
 				device mdio 0 on    # PHY address
 					ops m88e1512_ops
 				end