dmm/bm52x: introduce support for Brymen BM525s

dmm/bm52x: introduce support for Brymen BM525s

Introduce a bm52x DMM packet parser which is modelled after the bm86x
implementation, and hook it up to the serial-dmm driver. This supports
the live readings (real-time download) of the Brymen BM525s logging DMM.
The timing follows the vendor documentation (0.5s between requests, and
4.0s absolute timeout after request transmission). Reading previously
recorded data (memory data sets) unfortunately does not fit well into
the serial-dmm approach, and needs to get addressed differently later.