power/reset: xgene: Return -ENOMEM if out of memory

It is customary to return an error code of -ENOMEM if the system
is out of memory. Also, in that case, the infrastructure will report
an error, so it is unnecessary to report it again.

Cc: Loc Ho <lho@apm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>