ARM: dts: nxp: imx6: convert NVMEM content to layout syntax

Use cleaner (and non-deprecated) bindings syntax. See commit
bd912c99 ("dt-bindings: nvmem: layouts: add fixed-layout") for
details.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>