powerpc/doc: Update the README.srio-pcie-boot-corenet

1. Misalignment will be found in the doc/README.srio-pcie-boot-corenet
   file when the tabs are set to 8 characters. And the standard for
   u-boot should be 8 character tabs! So this issue should be amended.

2. Add a NOTE for the ENV parameters of the Slave.

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