feat(gd32): 优化GD32F470 SPI Flash初始化和UART0配置

主要修改：
1. SPI Flash初始化优化
   - 添加可配置的SPI Flash自动初始化选项(BSP_USING_SPI_FLASH)
   - 支持按SPI总线独立配置Flash初始化(BSP_USING_SPIx_FLASH)
   - 避免SPI Flash初始化与其他SPI设备(如OLED、WIFI)冲突
   - 添加SPI5 Flash支持
   - 修改drv_spi_flash.c，仅在明确配置的SPI总线上初始化Flash

2. UART0配置修复
   - 将UART0的AFIO默认值从AF1改为AF7
   - 修复串口无响应问题

这些修改使得用户可以更灵活地配置SPI Flash初始化，避免自动初始化导致的设备冲突问题。

Author: godmial

bsp/gd32/arm/libraries/gd32_drivers/drv_spi_flash.c

@@ -8,7 +8,7 @@
  * 2021-12-31     BruceOu      first implementation
  * 2023-06-03     CX           fixed sf probe error bug
  * 2024-05-30     godmial      refactor driver for multi-SPI bus auto-mount
- * 2024-12-XX     godmial         add configurable SPI Flash initialization
+ * 2025-11-28     godmial      add configurable SPI Flash initialization
  *                             Only initialize flash on SPI buses explicitly configured
  *                             via BSP_USING_SPIx_FLASH options to avoid conflicts
  *                             with other SPI devices (e.g., OLED, WIFI)