machine: add attiny85 pwm support (#5171)

* machine/attiny85: add PWM support for Timer0 and Timer1

Add complete PWM implementation for ATtiny85, supporting both Timer0
and Timer1 with their respective output channels:
- Timer0: 8-bit timer for pins PB0 (OC0A) and PB1 (OC0B)
- Timer1: 8-bit high-speed timer for pins PB1 (OC1A) and PB4 (OC1B)

Timer1 provides more flexible period control with configurable top value
(OCR1C) and extended prescaler options (1-16384), making it well-suited
for LED PWM control and other applications requiring variable frequencies.

Implements full PWM interface including Configure, SetPeriod, Channel,
Set, SetInverting, Top, Counter, and Period methods.

* machine/digispark: document PWM support on pins

Add documentation to the Digispark board file indicating which pins
support PWM output:
- P0 (PB0): Timer0 channel A
- P1 (PB1): Timer0 channel B or Timer1 channel A
- P4 (PB4): Timer1 channel B

Includes package comment explaining Timer0 vs Timer1 capabilities,
with Timer1 recommended for more flexible frequency control.

* machine/attiny85: optimize PWM prescaler lookups

Replace verbose switch statements with more efficient implementations:

- SetPeriod: Use bit shift (top >>= prescaler-1) instead of 15-case
  switch for dividing uint64 by power-of-2 prescaler values

- Period: Replace switch statements with compact uint16 lookup tables
  for both Timer0 and Timer1, casting to uint64 only when needed

This addresses review feedback about inefficient switch-based lookups.
On AVR, this approach is significantly smaller:
- Bit shifts for uint64 division: ~34 bytes vs ~140 bytes
- uint16 tables: 22 bytes code + 32/16 bytes data vs ~140 bytes
- Total savings: ~190 bytes (68% reduction)

* examples/pwm: add digispark support and smoketest

Add digispark.go configuration for PWM example using Timer1 with pins P1 (LED) and P4. Also add digispark PWM example to GNUmakefile smoketests.

---------

GNUmakefile

@@ -896,6 +896,8 @@ endif
 	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=digispark           examples/blinky1
 	@$(MD5SUM) test.hex
+	$(TINYGO) build -size short -o test.hex -target=digispark           examples/pwm
+	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=digispark -gc=leaking examples/blinky1
 	@$(MD5SUM) test.hex
 ifneq ($(XTENSA), 0)