target/arm: Report KVM's actual PSCI version to guest in dtb

When we're using KVM, the PSCI implementation is provided by the
kernel, but QEMU has to tell the guest about it via the device tree.
Currently we look at the KVM_CAP_ARM_PSCI_0_2 capability to determine
if the kernel is providing at least PSCI 0.2, but if the kernel
provides a newer version than that we will still only tell the guest
it has PSCI 0.2.  (This is fairly harmless; it just means the guest
won't use newer parts of the PSCI API.)

The kernel exposes the specific PSCI version it is implementing via
the ONE_REG API; use this to report in the dtb that the PSCI
implementation is 1.0-compatible if appropriate.  (The device tree
binding currently only distinguishes "pre-0.2", "0.2-compatible" and
"1.0-compatible".)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 20220224134655.1207865-1-peter.maydell@linaro.org