Add batch names to device tree files names and board type

With birch moving into place we should

• rename arch/arm64/boot/dts/freescale/imx8mq-librem5.dts to arch/arm64/boot/dts/freescale/imx8mq-librem5.dtsi
• add arch/arm64/boot/dts/freescale/imx8mq-librem5-aspen.dts and arch/arm64/boot/dts/freescale/imx8mq-librem5-birch.dts
• adjust the model in each dts accordingly, e.g. Purism Librem 5 Aspen
• adjust any further hardware chanegs

so we can distinguish the batches later on for updates from userspace and to identify customer devices and to e.g. pick the right uboot / dtb file when updating.