xen/arm: Remove __init from prototype

xen/arm: Remove __init from prototype

In Xen, it is common to add __init to the declaration and not the
prototype. Remove the few __init on some prototypes which allows to
avoid the inclusion of init.h in headers.

With these changes, init.h is now required to be included on some c
files. Also, add __init where it was missing in declaration.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>