Following is a gem from the MQX TWR-VF65GS10_a5 BSP (ver. 4.1.2). I am unsure whether this has been fixed in later versions, but on the off chance it is still there:
// clock CCM_CSCMR2 &= ~CCM_CSCMR2_RMII_CLK_SEL_MASK; CCM_CSCMR2 |= 0 << CCM_CSCMR2_RMII_CLK_SEL_SHIFT; // RMII_CLK // <------ CCM_CSCDR1 |= CCM_CSCDR1_RMII_CLK_EN_MASK; // enable RMII_CLK_EN
The extract is from where the IOMUX is set for the ENET/RMII (init_gpio.c::_bsp_enet_io_init).