x300: Cleaned up DAC ctrl and clock init logic

- DAC: Squashed configuration into 2 main operations: reset and reset_and_resync
- DAC: Put in sleep mode during configuration
- DAC: Synchronize only if streaming to more than one DAC
- DAC: Use falling edge sync mode
- DAC: Fixed power up/down settings
- DAC: Frontend sync failure is fatal
- Clocks: Refactored clock source change logic
- Clocks: Cleaned up init and lock-check sequence
examples: Fixed multi-channel ops
debug_prints_tool: New tool to plot the output of DEBUG_TXRX-enabled
output.
debug_prints_tool: Minor cleanup
e300: docs: Remove mention of E300 to not confuse people.

Signed-off-by: Moritz Fischer <moritz@ettus.com>
Merge branch 'maint'

Conflicts:
	host/examples/txrx_loopback_to_file.cpp
Merge branch 'master' into rfnoc-devel