dts: bindings: `zephyr,linker-region` property

Introduce optional `zephyr,linker-region` property which signifies that
the node should result in a linker memory region and what the name of
that region should be. Property added to compatibles likely to result
in a linker memory region; 'mmio-sram', 'arm,itcm`, `arm,dtcm`,
`nxp,imx-itcm`, `nxp,imx-dtcm` and `fixed-partitions`.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>