powerpc/corenet_ds: Get rid of the CONFIG_SRIOBOOT_SLAVE_PORTx macro

When compile the slave image for boot from SRIO, no longer need to
specify which SRIO port it will boot from. The code will get this
information from RCW and then finishes corresponding configurations.

This has the following advantages:
	1. No longer need to rebuild an image when change the SRIO port for
	   boot from SRIO, just rewrite the new RCW with selected port,
	   then the code will get the port information by reading new RCW.
	2. It will be easier to support other boot location options, for
	   example, boot from PCIE.

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