powerpc/32s: Allow deselecting CONFIG_PPC_FPU on mpc832x

The e300c2 core which is embedded in mpc832x CPU doesn't have
an FPU.

Make it possible to not select CONFIG_PPC_FPU when building a
kernel dedicated to that target.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/fcdc60d85baf80eaa0a7f3261d9d889282068216.1597770847.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_book3s_32.S

@@ -394,6 +394,7 @@ Alignment:
 	. = 0x800
 	DO_KVM  0x800
 FPUnavailable:
+#ifdef CONFIG_PPC_FPU
 BEGIN_FTR_SECTION
 /*
  * Certain Freescale cores don't have a FPU and treat fp instructions
@@ -407,6 +408,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_FPU_UNAVAILABLE)
 	b	fast_exception_return
 1:	addi	r3,r1,STACK_FRAME_OVERHEAD
 	EXC_XFER_LITE(0x800, kernel_fp_unavailable_exception)
+#else
+	b 	ProgramCheck
+#endif
 
 /* Decrementer */
 	EXCEPTION(0x900, Decrementer, timer_interrupt, EXC_XFER_LITE)

arch/powerpc/platforms/Kconfig.cputype

@@ -32,7 +32,7 @@ choice
 config PPC_BOOK3S_6xx
 	bool "512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx"
 	select PPC_BOOK3S_32
-	select PPC_FPU
+	imply PPC_FPU
 	select PPC_HAVE_PMU_SUPPORT
 	select PPC_HAVE_KUEP
 	select PPC_HAVE_KUAP
@@ -222,9 +222,16 @@ config PPC_FPU_REGS
 	bool
 
 config PPC_FPU
-	bool
+	bool "Support for Floating Point Unit (FPU)" if PPC_MPC832x
 	default y if PPC64
 	select PPC_FPU_REGS
+	help
+	  This must be enabled to support the Floating Point Unit
+	  Most 6xx have an FPU but e300c2 core (mpc832x) don't have
+	  an FPU, so when building an embedded kernel for that target
+	  you can disable FPU support.
+
+	  If unsure say Y.
 
 config FSL_EMB_PERFMON
 	bool "Freescale Embedded Perfmon"