arm: export platform_op XENPF_settime64

arm: export platform_op XENPF_settime64

Call update_domain_wallclock_time at domain initialization.
Set time_offset_seconds to the number of seconds between physical boot
and domain initialization: it is going to be used to get/set the
wallclock time.
Add time_offset_seconds to system_time when before calling do_settime,
so that system_time actually accounts for all the time in nsec between
machine boot and when the wallclock was set.

Expose xsm_platform_op to ARM.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
CC: dgdegra@tycho.nsa.gov