powerpc/corenet_ds: Update README and README.srio-pcie-boot-corenet

Added descriptions about boot from PCIE in the files README and
doc/README.srio-pcie-boot-corenet, and changed the name of the
doc/README.srio-boot-corenet to doc/README.srio-pcie-boot-corenet.

Signed-off-by: Liu Gang <Gang.Liu@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>