boards: arm: add WeAct STM32F446 Core board

Add support for WeAct STM32F446 Core board featuring:
- STM32F446RET6 MCU (ARM Cortex-M4F @ 180 MHz)
- 512KB Flash, 128KB SRAM
- 8MHz HSE crystal oscillator
- USB Type-C connector with DFU support
- User LED (PB2) and button (PC13)
- MicroSD card slot (SDIO)
- SWD debug header
- 30×2 pin headers exposing GPIO pins

Peripherals supported:
- 4x USART/UART
- 4x I2C
- 4x SPI/I2S
- 2x CAN
- 3x 12-bit ADC
- 2x 12-bit DAC
- USB OTG FS
- SDIO interface

Tested with samples/basic/blinky and samples/hello_world.

Signed-off-by: Siratul Islam <sirat4757@gmail.com>
Signed-off-by: heronet <sirat4757@gmail.com>