arm: extend pvclock_wall_clock with sec_hi

The hypervisor actually exposes an additional field to struct
pvclock_wall_clock, with the high 32 bit seconds.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>