board: siemens: iot2050: Generalize the fdt fixup

board: siemens: iot2050: Generalize the fdt fixup

The fdt fixup logic actually also applies to other possible variants who
also have device tree overlays. So generalize this part by extracting
it from the m.2 specific function and make it a standalone one.

Since now we only have M.2 variant consuming the overlay, it may not
have immediate effect for other variant, however this makes the future
variant more easier to apply fdt fixups.

Signed-off-by: Baocheng Su <baocheng.su@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>