dtlib: move some internal code around

Refactor the file parsing methods for readability by moving the
_parse_header() and _parse_memreserves() calls from _parse_dt() to
_parse_file(). The header and memreserves are not part of the 'tree'
part of the devicetree; now that we have a dedicated _parse_file()
helper, it makes more sense to me to have them there.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>