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.