siglent-sds: initial driver implementation for Siglent SDS

siglent-sds: initial driver implementation for Siglent SDS

Implement a first version of the driver for the Siglent SDS1000 and
SDS2000 oscilloscopes.

[ gsi: This commit corresponds to git 0228126017e6 of marchelh's tree,
  I adjusted the source to closer match the project's coding style. The
  conversion was verified by this command:

  $ git diff --word-diff=color -w -b <branch>:<dir> <branch>:<dir>

  Changes include: Whitespace adjustment, dead code removal, separation
  of variable declaration and assignment, alpha-sorted includes. Line
  length was not addressed and not every location got adjusted, to keep
  the diff minimal and to reduce effort during review of this version. ]