powerpc/b4860qds: Enable master module for boot from SRIO and PCIE

B4860QDS can support the feature of Boot from SRIO/PCIE, and the macro
"CONFIG_SRIO_PCIE_BOOT_MASTER" will enable the master module of this feature
when building the u-boot image.

You can get some description about this macro in README file, and for more
information about the feature of Boot from SRIO/PCIE, please refer to the
document doc/README.srio-pcie-boot-corenet.

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