net: lwm2m: fix registration content format and use plain/text

1. According to the specification 5.3.1, it's a MUST to specify
   (1) content format: app link format (2) supported lwm2m version.
   Also, we should use text/plain instead of LWM2M's (obsolete).
2. Use LWM2M_FORMAT_OMA_TLV as default accept format when accept option
   is not given from the caller for TLV is a MUST have in LwM2M spec and
   it can deals w/ multiple resources read

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