MIPS: Add definitions of SegCtl registers and use them

The SegCtl registers are standard for MIPSr3..MIPSr5. Add definitions of
these registers and use them rather than constants

Signed-off-by: Matt Redfearn <[e-mail]>
Cc: Joshua Kinard <[e-mail]>
Cc: James Hogan <[e-mail]>
Cc: Chris Packham <[e-mail]>
Cc: Paul Burton <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/13290/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/include/asm/mach-malta/kernel-entry-init.h

···
 		(0 << MIPS_SEGCFG_PA_SHIFT) |				\
 		(1 << MIPS_SEGCFG_EU_SHIFT)) << 16)
 	or	t0, t2
-	mtc0	t0, $5, 2
+	mtc0	t0, CP0_SEGCTL0
 
 	/* SegCtl1 */
 	li      t0, ((MIPS_SEGCFG_MUSUK << MIPS_SEGCFG_AM_SHIFT) |	\
···
 		(0 << MIPS_SEGCFG_PA_SHIFT) |				\
 		(1 << MIPS_SEGCFG_EU_SHIFT)) << 16)
 	ins	t0, t1, 16, 3
-	mtc0	t0, $5, 3
+	mtc0	t0, CP0_SEGCTL1
 
 	/* SegCtl2 */
 	li	t0, ((MIPS_SEGCFG_MUSUK << MIPS_SEGCFG_AM_SHIFT) |	\
···
 		(4 << MIPS_SEGCFG_PA_SHIFT) |				\
 		(1 << MIPS_SEGCFG_EU_SHIFT)) << 16)
 	or	t0, t2
-	mtc0	t0, $5, 4
+	mtc0	t0, CP0_SEGCTL2
 
 	jal	mips_ihb
 	mfc0    t0, $16, 5

arch/mips/include/asm/mipsregs.h

···
 #define CP0_CONF $3
 #define CP0_CONTEXT $4
 #define CP0_PAGEMASK $5
+#define CP0_SEGCTL0 $5, 2
+#define CP0_SEGCTL1 $5, 3
+#define CP0_SEGCTL2 $5, 4
 #define CP0_WIRED $6
 #define CP0_INFO $7
 #define CP0_HWRENA $7, 0