xen/arm: Introduce a initcall to update cpu_hwcaps by serror_op

xen/arm: Introduce a initcall to update cpu_hwcaps by serror_op

In the later patches of this series, we want to use the alternative
patching framework to avoid synchronizing serror_op in every entries
and exits. So we define a new cpu feature "SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT"
for serror_op. When serror_op is not equal to SERROR_DIVERSE, this
feature will be set to cpu_hwcaps.

Currently, the default serror_op is SERROR_DIVERSE, if we want to
change the serror_op value we have to place the serror parameter
in command line. It seems no problem to update cpu_hwcaps directly
in the serror parameter parsing function.

While the default option will be diverse today, this may change in the
future. So we introduce this initcall to guarantee the cpu_hwcaps can be
updated no matter the serror parameter is placed in the command line
or not.

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