mach-snapdragon: handle platforms without PSCI support

Most MSM8916 devices shipped without PSCI support. The history is quite
nuanced (a good overview can be found in [1]), but the end result is
that the upstream DTs for this SoC pretend that PSCI exists, and it's
expected that the bootloader handles the case where it doesn't. This is
codified by the de-facto bootloader for MSM8916 devices, lk2nd [2].

So we handle it here by deleting the /psci node if we detect the absence
of PSCI. We need to do this early to ensure sysreset works correctly,
since the PSCI firmware driver is PRE_RELOC and binds the PSCI sysreset
driver.

Additionally, show_psci_version is updated to check that PSCI exists.
Currently this banner outputs "PSCI: 65535.65535" on devices without
PSCI support, which isn't very useful :)

[1]: https://github.com/msm8916-mainline/linux/issues/388
[2]: https://github.com/msm8916-mainline/lk2nd/blob/8183ea2/lk2nd/smp/spin-table/spin-table.c#L237

Signed-off-by: Sam Day <me@samcday.com>
Link: https://lore.kernel.org/r/20250127-qcom-handle-absent-psci-v1-1-e762f2db938c@samcday.com
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>

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