powerpc/corenet_ds: Slave reads ENV from master when boot from SRIO

When boot from SRIO, slave's ENV can be stored in master's memory space,
then slave can fetch the ENV through SRIO interface.

NOTE: Because the slave can not erase, write master's NOR flash by SRIO
	  interface, so it can not modify the ENV parameters stored in
	  master's NOR flash using "saveenv" or other commands.

Master needs to:
	1. Put the slave's ENV into it's own memory space.
	2. Set an inbound SRIO window covered slave's ENV stored in master's
	   memory space.
Slave needs to:
	1. Set a specific TLB entry in order to fetch ucode and ENV from master.
	2. Set a LAW entry with the TargetID SRIO1 or SRIO2 for ucode and ENV.

Signed-off-by: Liu Gang <Gang.Liu@freescale.com>
Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>