powerpc/boot: Change the compile macro for SRIO & PCIE boot master module

Currently, the SRIO and PCIE boot master module will be compiled into the
u-boot image if the macro "CONFIG_FSL_CORENET" has been defined. And this
macro has been included by all the corenet architecture platform boards.
But in fact, it's uncertain whether all corenet platform boards support
this feature.

So it may be better to get rid of the macro "CONFIG_FSL_CORENET", and add
a special macro for every board which can support the feature. This
special macro will be defined in the header file
"arch/powerpc/include/asm/config_mpc85xx.h". It will decide if the SRIO
and PCIE boot master module should be compiled into the board u-boot image.

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