phy: freescale: imx8m-pcie: fix pcie link-up instability

Leaving AUX_PLL_REFCLK_SEL at its reset default of AUX_IN (PLL clock)
proves to be more stable on the i.MX 8M Mini.

Fixes: 1aa97b00

 ("phy: freescale: pcie: Initialize the imx8 pcie standalone phy driver")

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Richard Zhu <hongxing.zhu@nxp.com>
Link: https://lore.kernel.org/r/20240322130646.1016630-2-marcel@ziswiler.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>