Transceiver52M: Move reference select from compile time to database

Enabling the external reference on UHD devices through the configure
time switch is awkward. Use a database variable "TRX.Reference" with
'0' or '1' value for internal and external references respectively.
Use internal reference is no entry is defined.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
7 files changed