arm64: Add SPE support

Add a runtime configurable for kvmtool to enable SPE (Statistical
Profiling Extensions) for each vcpu and to create a corresponding DT node.
SPE is enabled at runtime with the --spe option.

SPE is last to be initialized because it requires the VGIC to be
initialized, which is done late in the initialization process.

[ Andrew M: Add SPE to init features ]
[ Alexandru E: Reworked patch ]

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>