librem5-devkit: Enable CONFIG_ARM_IMX_CPUFREQ_DT
This is gets enabled here https://source.puri.sm/guido.gunther/linux-imx8/blob/imx8-5.x-drm/arch/arm64/configs/librem5-devkit_defconfig#L53 but for some reason doesn't end up in this tree. Signed-off-by: Guido Günther <guido.gunther@puri.sm>