powerpc/64e: Tie PPC_BOOK3E_64 to PPC_E500MC

The only 64-bit Book3E CPUs we support require the selection
of CONFIG_PPC_E500MC.

However our Kconfig allows configurating a kernel that has 64-bit
Book3E support, but without CONFIG_PPC_E500MC enabled. Such a kernel
would never boot, it doesn't know about any CPUs.

To fix this, force CONFIG_PPC_E500MC to be selected whenever we are
building a 64-bit Book3E kernel.

And add a test to detect future situations where cpu_specs is empty.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ae5d8b8b3ccc346e61d2ec729767f92766273f0b.1663606875.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/cputable.c

@@ -2018,6 +2018,8 @@ struct cpu_spec * __init identify_cpu(unsigned long offset, unsigned int pvr)
 	struct cpu_spec *s = cpu_specs;
 	int i;
 
+	BUILD_BUG_ON(!ARRAY_SIZE(cpu_specs));
+
 	s = PTRRELOC(s);
 
 	for (i = 0; i < ARRAY_SIZE(cpu_specs); i++,s++) {

arch/powerpc/platforms/Kconfig.cputype

@@ -108,6 +108,8 @@ config PPC_BOOK3S_64
 config PPC_BOOK3E_64
 	bool "Embedded processors"
 	select PPC_FSL_BOOK3E
+	select E500
+	select PPC_E500MC
 	select PPC_FPU # Make it a choice ?
 	select PPC_SMP_MUXED_IPI
 	select PPC_DOORBELL