powerpc/mm: Add mask of always present MMU features

On the same principle as commit 773edead ("powerpc/mm: Add mask
of possible MMU features"), add mask for MMU features that are
always there in order to optimise out dead branches.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4943775fbe91885eb3e09133b093aaf62e55c715.1603348103.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/mmu.h

@@ -206,8 +206,30 @@ enum {
 		0,
 };
 
+#ifdef CONFIG_PPC_8xx
+#define MMU_FTRS_ALWAYS		MMU_FTR_TYPE_8xx
+#endif
+#ifdef CONFIG_40x
+#define MMU_FTRS_ALWAYS		MMU_FTR_TYPE_40x
+#endif
+#ifdef CONFIG_PPC_47x
+#define MMU_FTRS_ALWAYS		MMU_FTR_TYPE_47x
+#elif defined(CONFIG_44x)
+#define MMU_FTRS_ALWAYS		MMU_FTR_TYPE_44x
+#endif
+#if defined(CONFIG_E200) || defined(CONFIG_E500)
+#define MMU_FTRS_ALWAYS		MMU_FTR_TYPE_FSL_E
+#endif
+
+#ifndef MMU_FTRS_ALWAYS
+#define MMU_FTRS_ALWAYS		0
+#endif
+
 static inline bool early_mmu_has_feature(unsigned long feature)
 {
+	if (MMU_FTRS_ALWAYS & feature)
+		return true;
+
 	return !!(MMU_FTRS_POSSIBLE & cur_cpu_spec->mmu_features & feature);
 }
 
@@ -236,6 +258,9 @@ static __always_inline bool mmu_has_feature(unsigned long feature)
 	}
 #endif
 
+	if (MMU_FTRS_ALWAYS & feature)
+		return true;
+
 	if (!(MMU_FTRS_POSSIBLE & feature))
 		return false;