Introduce CONFIG_PARAVIRT and PARAVIRT_TIME_ACCOUNTING on ARM.

The only paravirt interface supported is pv_time_ops.steal_clock, so no
runtime pvops patching needed.

This allows us to make use of steal_account_process_tick for stolen
ticks accounting.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: NChristopher Covington <cov@codeaurora.org>
Acked-by: NIan Campbell <ian.campbell@citrix.com>
Acked-by: NRussell King <linux@arm.linux.org.uk>