xen/arm: Detect silicon revision and set cap bits accordingly

xen/arm: Detect silicon revision and set cap bits accordingly

After each CPU has been started, we iterate through a list of CPU
errata to detect CPUs which need from hypervisor code patches.

For each bug there is a function which checks if that a particular CPU is
affected. This needs to be done on every CPU to cover heterogenous
systems properly.

If a certain erratum has been detected, the capability bit will be set.
In the case the erratum requires code patching, this will be triggered
by the call to apply_alternatives.

The code is based on the file arch/arm64/kernel/cpu_errata.c in Linux
v4.6-rc3.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>