powerpc: FA_DUMP depends on KEXEC

If you try and build the FA_DUMP code with CONFIG_KEXEC=n, you see
errors such as the following:

arch/powerpc/kernel/fadump.c
  408:2: error: 'crashing_cpu' undeclared (first use in this function)
  410:2: error: implicit declaration of function 'crash_save_vmcoreinfo'
  513:22: error: storage size of 'prstatus' isn't known
  520:2: error: implicit declaration of function 'elf_core_copy_kernel_regs'
  521:36: error: 'KEXEC_CORE_NOTE_NAME' undeclared (first use in this function)
  624:49: error: 'note_buf_t' undeclared (first use in this function)
  872:2: error: implicit declaration of function 'paddr_vmcoreinfo_note'
  874:18: error: 'vmcoreinfo_max_size' undeclared (first use in this function)

This is because although FA_DUMP doesn't use kexec as the actual reboot
mechanism, it does use parts of the kexec code to assemble/disassemble
the crash image.
Signed-off-by: NMichael Ellerman <michael@ellerman.id.au>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/Kconfig

@@ -407,7 +407,7 @@ config CRASH_DUMP
 
 config FA_DUMP
 	bool "Firmware-assisted dump"
-	depends on PPC64 && PPC_RTAS && CRASH_DUMP
+	depends on PPC64 && PPC_RTAS && CRASH_DUMP && KEXEC
 	help
 	  A robust mechanism to get reliable kernel crash dump with
 	  assistance from firmware. This approach does not use kexec,