net: lwm2m: Align float handling with specification

According to the specificaion, resources are not predefined to use 32 or
64 bit floating point numbers, but should rather accept any of them (as
indicated by the size of the TLV in the message). This lead to issues
for instance with Eclipse Leshan LWM2M server, where Leshan sent 64-bit
value, while Zephyr expected 32-bit, making it impossible to write
the resource.

Therefore, unify the float usage to 32-bit float representation and fix
the TLV parsing functions, to accept values sent as either 32 or 64 bit
float.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>