ARM: imx: build suspend-imx6.S with arm instruction set

When the kernel is configured to use the Thumb-2 instruction set
"suspend-to-memory" fails to resume. Observed on a Colibri iMX6ULL
(i.MX 6ULL) and Apalis iMX6 (i.MX 6Q).

It looks like the CPU resumes unconditionally in ARM instruction mode
and then chokes on the presented Thumb-2 code it should execute.

Fix this by using the arm instruction set for all code in
suspend-imx6.S.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Fixes: df595746 ("ARM: imx: add suspend in ocram support for i.mx6q")
Acked-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>