use simple 8-bit counter for LED PWM

Smol Watch Project/My Project/hpl/doc_lite/tc.rst

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+=========
+TC driver
+=========
+The TC consists of a counter, a prescaler, compare/capture channels and control logic. The counter can be set to count events, or it can be configured to count clock pulses. The counter, together with the compare/capture channels, can be configured to timestamp input events, allowing capture of frequency and pulse width. It can also perform waveform generation, such as frequency generation and pulse-width modulation (PWM) 
+
+The timer/counter is clocked by the peripheral clock with optional prescaling or from the event system.
+
+Features
+--------
+* Initialization
+
+Applications
+------------
+* Frequency Generation
+* Single-slope PWM (pulse width modulation)
+* Dual-slope PWM
+* Count on event
+* Quadrature decoding
+
+Dependencies
+------------
+* CLK for clock
+* CPUINT/PMIC for Interrupt
+* EVSYS for events
+* UPDI/PDI/JTAG for debug
+* PORT for Waveform Generation
+
+Concurrency
+-----------
+N/A
+
+Limitations
+-----------
+N/A
+
+Knows issues and workarounds
+----------------------------
+N/A
+