MIPS: Avoid magic numbers probing kscratch_mask

The decode_config4() function reads kscratch_mask from
CP0_Config4.KScrExist using a hard coded shift and mask. We already have
a definition for the mask in mipsregs.h, so add a definition for the
shift and make use of them.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13227/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/mipsregs.h

@@ -615,7 +615,8 @@
 #define MIPS_CONF4_MMUEXTDEF_MMUSIZEEXT (_ULCAST_(1) << 14)
 #define MIPS_CONF4_MMUEXTDEF_FTLBSIZEEXT	(_ULCAST_(2) << 14)
 #define MIPS_CONF4_MMUEXTDEF_VTLBSIZEEXT	(_ULCAST_(3) << 14)
-#define MIPS_CONF4_KSCREXIST	(_ULCAST_(255) << 16)
+#define MIPS_CONF4_KSCREXIST_SHIFT	(16)
+#define MIPS_CONF4_KSCREXIST	(_ULCAST_(255) << MIPS_CONF4_KSCREXIST_SHIFT)
 #define MIPS_CONF4_VTLBSIZEEXT_SHIFT	(24)
 #define MIPS_CONF4_VTLBSIZEEXT	(_ULCAST_(15) << MIPS_CONF4_VTLBSIZEEXT_SHIFT)
 #define MIPS_CONF4_AE		(_ULCAST_(1) << 28)

arch/mips/kernel/cpu-probe.c

@@ -801,7 +801,8 @@ static inline unsigned int decode_config4(struct cpuinfo_mips *c)
 		}
 	}
 
-	c->kscratch_mask = (config4 >> 16) & 0xff;
+	c->kscratch_mask = (config4 & MIPS_CONF4_KSCREXIST)
+				>> MIPS_CONF4_KSCREXIST_SHIFT;
 
 	asid_mask = MIPS_ENTRYHI_ASID;
 	if (config4 & MIPS_CONF4_AE)