arm: add dummy implementation of arch_curr_cpu

This is required by drivers which query the current cpu,
at this moment there is no arm aarch32 that use smp,
so it seems safe to consider that the current cpu is
always the first one.

This patch enable the use of the GICv3 driver on
ARM 32bits cpu.

Signed-off-by: Julien Massot <julien.massot@iot.bzh>