powerpc/64: Set DSCR default initially from SPR

Take the DSCR value set by firmware as the dscr_default value,
rather than zero.

POWER9 recommends DSCR default to a non-zero value.

Signed-off-by: From: Nicholas Piggin <npiggin@gmail.com>
[mpe: Make record_spr_defaults() __init]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/setup.h

@@ -45,6 +45,12 @@ void emergency_stack_init(void);
 static inline void emergency_stack_init(void) { };
 #endif
 
+#ifdef CONFIG_PPC64
+void record_spr_defaults(void);
+#else
+static inline void record_spr_defaults(void) { };
+#endif
+
 /*
  * Having this in kvm_ppc.h makes include dependencies too
  * tricky to solve for setup-common.c so have it here.