Revert "librem5: Enable ARM_PSCI_CPUIDLE"

This reverts commit 9f61faa653cb78df9ee4547c7c617e59097c242a.

Causes boot hang at

[    1.302069] loop: module loaded
[    1.306953] libphy: Fixed MDIO Bus: probed
[    1.313435] snvs_rtc 30370000.snvs:snvs-rtc-lp: registered as rtc0
[    1.319991] i2c /dev entries driver
[    1.328004] device-mapper: ioctl: 4.41.0-ioctl (2019-09-16) initialised: dm-devel@redhat.com
[    1.338525] sdhci: Secure Digital Host Controller Interface driver
[    1.344741] sdhci: Copyright(c) Pierre Ossman
[    1.349163] sdhci-pltfm: SDHCI platform and OF driver helper
...hang...