net: lwm2m: only method GET is allowed for .well-known/core

".well-known/core" is mainly used with method GET for performing the
resource discovery (RFC 6690). Since we are implementing a LwM2M client
and is not implement a resource directory which allow others to do the
resource registration (POST to .well-known/core). Only GET method is
allowed for the usage. Report 4.5 (Method Not Allowed) if other methods
are requested.

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