powerpc/dts/fsl: add crypto node alias for B4

crypto node alias is needed by U-boot to identify the node and
perform fix-ups, like adding "fsl,sec-era" property.

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>