mtd: chips/map_rom.c: implement point and unpoint methods

This will allow for the removal of the get_unmapped_area method later.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Chris Brandt <chris.brandt@renesas.com>
[rw: fixed build]
Signed-off-by: Richard Weinberger <richard@nod.at>