mach-imx/mach-imx6ul.c: proper constness with __initconst

Both the pointer array and the pointed data have to be const when using
__initconst to be correct.  This also fixes LTO builds that otherwise
fail with section mismatch errors.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>