xen/arm: Initialize PERCPU variables at the beginning of start_xen

xen/arm: Initialize PERCPU variables at the beginning of start_xen

PERCPU variables rely on HTPIDR (TPIDR_EL2) which is in an unknown state when
a processor boot.

For the boot CPU, the first use of PERCPU is in setup_pagetables. So
initialize PERCPU and set the processor ID before.

Bamvor Jian Zhang observed this failure on the sun6i processor which does not
initialise HTPIDR and contributed a very similar patch.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
[ ijc -- added last para of commit message ]