ARM: smp: Make axxia_pen_release volatile

commit 1c6951eed24d44879f696c25e564cc65c5bf544a from
https://github.com/axxia/linux-yocto.git v5.10/standard/axxia-dev/base.

The old pen_release was declared volatile in kernel v4.19 and
this is also how it is impleneted for other arm achitectures.

Signed-off-by: Mikael Beckius <mikael.beckius@windriver.com>
Signed-off-by: Daniel Dragomir <Daniel.Dragomir@windriver.com>
Signed-off-by: Liwei Song <liwei.song@windriver.com>