[POWERPC] Introduce _SYSDEV_ATTR

Introduce _SYSDEV_ATTR(), to be used to just define the struct, and not a
named variable with the attribute. Useful for arrays of sysdev_attributes.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>