xen/arm: ioremap_attr: return NULL is __vmap failed

xen/arm: ioremap_attr: return NULL is __vmap failed

Most of ioremap_* caller check if ioremap returns NULL. Actually, if the
physical address is non-aligned, Xen will return the pointer given by
__vmap plus the offset in the page. So if ioremap_* fails, the caller
will retrieve an non-NULL address and continue as if there was no error.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>