3 days and no response useful response (again).
So I solved the problem, after configuring ENET PLL as stated on reference manual, and set the IOMUX to redirect the generated clock to the phy, now I able the 50MHz clock to use in RMII mode and got the phy fully working, to help others with same question, follow the code snippet below before configure the ethernet:
const clock_enet_pll_config_t config = { .enableClkOutput0 = true, .enableClkOutput1 = false, .enableClkOutput2 = false, .loopDivider0 = 0x01, .loopDivider1 = 0x01 };
/* configure ethernet pin_muxing and clocking*/
CLOCK_InitEnetPll(&config);
IOMUXC_EnableMode(IOMUXC_GPR, kIOMUXC_GPR_ENET1TxClkOutputDir, true);
BOARD_InitENET();
Best.
Felipe