powerpc/time: Remove ifdef in get_dec() and set_dec()

Move SPRN_PIT definition in reg.h.

This allows to remove ifdef in get_dec() and set_dec() and
makes them more readable.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3c9a6eb0fc040868ac59be66f338d08fd017668d.1601549945.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/reg.h

@@ -521,6 +521,8 @@
 #define SPRN_TSCR	0x399	/* Thread Switch Control Register */
 
 #define SPRN_DEC	0x016		/* Decrement Register */
+#define SPRN_PIT	0x3DB		/* Programmable Interval Timer (40x/BOOKE) */
+
 #define SPRN_DER	0x095		/* Debug Enable Register */
 #define DER_RSTE	0x40000000	/* Reset Interrupt */
 #define DER_CHSTPE	0x20000000	/* Check Stop */

arch/powerpc/include/asm/reg_booke.h

@@ -174,7 +174,6 @@
 #define SPRN_L1CSR1	0x3F3	/* L1 Cache Control and Status Register 1 */
 #define SPRN_MMUCSR0	0x3F4	/* MMU Control and Status Register 0 */
 #define SPRN_MMUCFG	0x3F7	/* MMU Configuration Register */
-#define SPRN_PIT	0x3DB	/* Programmable Interval Timer */
 #define SPRN_BUCSR	0x3F5	/* Branch Unit Control and Status */
 #define SPRN_L2CSR0	0x3F9	/* L2 Data Cache Control and Status Register 0 */
 #define SPRN_L2CSR1	0x3FA	/* L2 Data Cache Control and Status Register 1 */

arch/powerpc/include/asm/time.h

@@ -100,11 +100,10 @@ static inline void set_tb(unsigned int upper, unsigned int lower)
  */
 static inline u64 get_dec(void)
 {
-#if defined(CONFIG_40x)
-	return (mfspr(SPRN_PIT));
-#else
-	return (mfspr(SPRN_DEC));
-#endif
+	if (IS_ENABLED(CONFIG_40x))
+		return mfspr(SPRN_PIT);
+
+	return mfspr(SPRN_DEC);
 }
 
 /*
@@ -114,14 +113,12 @@ static inline u64 get_dec(void)
  */
 static inline void set_dec(u64 val)
 {
-#if defined(CONFIG_40x)
-	mtspr(SPRN_PIT, (u32) val);
-#else
-#ifndef CONFIG_BOOKE
-	--val;
-#endif
-	mtspr(SPRN_DEC, val);
-#endif /* not 40x */
+	if (IS_ENABLED(CONFIG_40x))
+		mtspr(SPRN_PIT, (u32)val);
+	else if (IS_ENABLED(CONFIG_BOOKE))
+		mtspr(SPRN_DEC, val);
+	else
+		mtspr(SPRN_DEC, val - 1);
 }
 
 static inline unsigned long tb_ticks_since(unsigned long tstamp)