crypto: caam - fix ERA retrieval function

SEC ERA has to be retrieved by reading the "fsl,sec-era" property
from the device tree. This property is updated/filled in by
u-boot.

Signed-off-by: Alex Porosanu <alexandru.porosanu@freescale.com>
Reviewed-by: Horia Geanta <horia.geanta@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>