machine/stm32: add STM32U5 GPIO, pin interrupts, and UART support

Add machine-level support for STM32U5 family: GPIO ports A-I with clock
enables via AHB2ENR1, EXTI pin interrupts (individual EXTI0-15 IRQs),
timer definitions (TIM1-8, TIM15-17), RNG, and alternate function clock
enables for all peripherals.

Add UART support with STM32U585 baud rate and register configuration.
Add arduino-uno-q board pin definitions and USART2 serial.

Update shared build tags: gpio_revb, gpio_reva, exti_exti, exti_syscfg.

src/machine/machine_stm32u585.go

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+package tmp
+//go:build stm32u585
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+}	uart.txEmptyFlag = stm32.USART_ISR_TXE	uart.statusReg = &uart.Bus.ISR	uart.txReg = &uart.Bus.TDR	uart.rxReg = &uart.Bus.RDRfunc (uart *UART) setRegisters() {// Register names vary by ST processor, these are for STM U5}	return CPUFrequency() / baudRatefunc (uart *UART) getBaudRateDivisor(baudRate uint32) uint32 {// NOTE: keep this in sync with the runtime/runtime_stm32u5.go clock init code// UART baudrate calc based on the bus and clockspeed}	config.RX.ConfigureAltFunc(PinConfig{Mode: PinModeUARTRX}, uart.RxAltFuncSelector)	config.TX.ConfigureAltFunc(PinConfig{Mode: PinModeUARTTX}, uart.TxAltFuncSelector)	// enable the alternate functions on the TX and RX pins	}		stm32.PWR.SetSVMCR_IO2SV(1)		// Enable VDDIO2 power supply for PGx pins	if config.RX.getPort() == stm32.GPIOG || config.TX.getPort() == stm32.GPIOG {func (uart *UART) configurePins(config UARTConfig) {// Configure the UART.//---------- UART related codeconst APB2_TIM_FREQ = 160e6 // 160MHzconst APB1_TIM_FREQ = 160e6 // 160MHz// and clock frequencies// in sync with any changes to runtime package which configures the oscillators// Internal use: configured speed of the APB1 and APB2 timers, this should be kept}	return 160000000func CPUFrequency() uint32 {)	"device/stm32"import (package machine