ppc: Build in all three of powermac, PREP and CHRP support

This reverts commit da0825fd, making
it so that if you select CONFIG_PPC_MULTIPLATFORM you get support
for PMAC, PREP and CHRP built in.

The reason for not allowing PMAC, PREP and CHRP to be selected
individually for ARCH=ppc is that there is too much interdependency
between them in the platform support code.  For example, CHRP uses
the PMAC nvram code.

Configuring with ARCH=powerpc does allow you to select support for
PMAC and CHRP separately.  Support for PREP is not there yet but
should be there soon.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/ppc/Kconfig

@@ -767,14 +767,14 @@ config CPM2
 	  on it (826x, 827x, 8560).
 
 config PPC_CHRP
-	bool "  Common Hardware Reference Platform (CHRP) based machines"
+	bool
 	depends on PPC_MULTIPLATFORM
 	select PPC_I8259
 	select PPC_INDIRECT_PCI
 	default y
 
 config PPC_PMAC
-	bool "  Apple PowerMac based machines"
+	bool
 	depends on PPC_MULTIPLATFORM
 	select PPC_INDIRECT_PCI
 	default y
@@ -785,7 +785,7 @@ config PPC_PMAC64
 	default y
 
 config PPC_PREP
-	bool "  PowerPC Reference Platform (PReP) based machines"
+	bool
 	depends on PPC_MULTIPLATFORM
 	select PPC_I8259
 	select PPC_INDIRECT_PCI