configs/freescale_imx6qsabresd: rename imx6 to imx6q

This defconfig is specific to the SABRESD board equipped with the iMX6Q SoC.
Rename it for clarity in order to avoid confusion with the upcoming
defconfig for the iMX6DL version.

Suggested by Gary Bisson.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Gilles Talis <gilles.talis@gmail.com>
Cc: Gary Bisson <bisson.gary@gmail.com>
Cc: Vincent Stehlé <vincent.stehle@freescale.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>