net: lwm2m: add write-attribute WRITE support

Implement write-attribute on obj/obj_inst/res according to LwM2M spec
20170704-A, sec 5.1.2. Support pmin/pmax/st/gt/lt parameters on WRITE
operation.

The basic idea is to add sys_slist_t to obj/obj_inst/res structure.
And attach struct lwm2m_attr to the list when attributes are written
from server side (implement lwm2m_write_attr_handler accordingly)

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>