powerpc/fsl_booke/kaslr: support nokaslr cmdline parameter

One may want to disable kaslr when boot, so provide a cmdline parameter
'nokaslr' to support this.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Diana Craciun <diana.craciun@nxp.com>
Tested-by: Diana Craciun <diana.craciun@nxp.com>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>