kvm tools: arm: add support for PSCI firmware in place of spin-tables

ARM has recently published a document describing a firmware interface
for CPU power management, which can be used for booting secondary cores
on an SMP platform, amongst other things. As part of the mach-virt
upstreaming for the kernel (that is, the virtual platform targetted by
kvmtool), it was suggested that we use this interface instead of the
current spin-table based approach.

This patch implements PSCI support in kvmtool for ARM, removing a fair
amount of code in the process.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>