ARM: dts: imx6qdl: don't configure reserved pad settings

Several dts files set a bit in the SPEED field for pads
RGMII_{R,T}{XC,D0,D1,D2,D3,X_CTL}, but that doesn't exist. Writing there
doesn't have an effect and the bit reads as zero.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>