Initial commit: LSPi (LiangShanPi) project

GD32F470ZGT6 based project with full peripheral support:
- SDRAM (W9825G6KH) via EXMC
- LCD NT35510 (480x800) via EXMC NOR/PSRAM
- Flash (W25Q64) via SPI
- UART (USART0) debug console
- LED indicators
- CMSIS-DAP debug interface
- CMake + ARM GCC toolchain build system

drv/uart/uart_driver.h

@@ -0,0 +1,43 @@
+#ifndef __UART_DRIVER_H__
+#define __UART_DRIVER_H__
+
+#include <cstdint>
+#include "bus_core.h"
+
+enum class UartPort : uint8_t {
+    _0 = 0,
+    _1, _2, _3, _4, _5, _6,
+    MAX
+};
+
+struct UartConfig {
+    uint32_t baudrate = 115200;
+    uint8_t data_bits = 8;
+    uint8_t stop_bits = 1;
+    uint8_t parity = 0;
+};
+
+class UartBus {
+public:
+    explicit UartBus(UartPort port);
+    ~UartBus();
+
+    UartBus(const UartBus &) = delete;
+    UartBus &operator=(const UartBus &) = delete;
+
+    RetCode init(const UartConfig &config);
+    RetCode deinit();
+    RetCode send_byte(uint8_t data);
+    RetCode send_data(const uint8_t *data, uint32_t len);
+    RetCode receive_byte(uint8_t *data, uint32_t timeout_ms);
+    RetCode irq_handler();
+
+    static UartBus &default_instance();
+
+private:
+    UartPort port_;
+    UartConfig config_;
+    bool initialized_ = false;
+};
+
+#endif