rp2350: add pll generalized solution; fix ADC handles; pwm period fix

src/machine/machine_rp2_2350a.go

@@ -0,0 +1,14 @@
+//go:build rp2350 && !rp2350b
+
+package machine
+
+// Analog pins on RP2350a.
+const (
+	ADC0 Pin = GPIO26
+	ADC1 Pin = GPIO27
+	ADC2 Pin = GPIO28
+	ADC3 Pin = GPIO29
+
+	// fifth ADC channel.
+	thermADC = 30
+)