自己从固件usbfs例子中找到了如何配置时钟：主频设为72MHz, 通过 usb_rcu_init 函数可以设置 usb主频为 48MHz
//此段拷贝于 GD32F105 固件的 USBFS 例子
// CK_PLL = (CK_PREDIV0) * 18 = 72 MHz
RCU_CFG0 &= ~(RCU_CFG0_PLLMF | RCU_CFG0_PLLMF_4);
RCU_CFG0 |= (RCU_PLLSRC_HXTAL | RCU_PLL_MUL18);
// CK_PREDIV0 = (CK_HXTAL)/5 *8 /10 = 4 MHz
RCU_CFG1 &= ~(RCU_CFG1_PREDV0SEL | RCU_CFG1_PLL1MF | RCU_CFG1_PREDV1 | RCU_CFG1_PREDV0);
RCU_CFG1 |= (RCU_PREDV0SRC_CKPLL1 | RCU_PLL1_MUL8 | RCU_PREDV1_DIV5 | RCU_PREDV0_DIV10);
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