ARM: imx: Kconfig: Fix indentation

The convention for indentation seems to be a single tab. Help text is
further indented by an additional two whitespaces. Fix the lines that
violate these rules.

While at it, replace separator tabs with whitespaces.
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>