ARM: dts: imx6qdl-tqma6: add ERR006687 hardware workaround for "a" variant

The TQMa6x "a" has a workaround for ERR006687 implemented in hardware.
Add the required pinmuxing and related setup to make use of this.

As board DTS files based on the TQMa6x SoMs will define their own
pinmuxing for the FEC ethernet controller as well, we can't apply this
pin group unconditionally; instead, it is the responsibility of the
board DTs derived from imx6qdl-tqma6a.dtsi to include this group.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>