xtensa: fix ibreakenable register update

Only set the register when there is at least one ibreak register,
otherwise the build fails:
	arch/xtensa/kernel/head.S:105: Error: invalid register 'ibreakenable'
	for 'wsr' instruction
	arch/xtensa/platforms/iss/setup.c:67: Error: invalid register
	'ibreakenable' for 'wsr' instruction

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/kernel/head.S

@@ -86,7 +86,9 @@ ENTRY(_startup)
 	/* Clear debugging registers. */
 
 #if XCHAL_HAVE_DEBUG
+#if XCHAL_NUM_IBREAK > 0
 	wsr	a0, ibreakenable
+#endif
 	wsr	a0, icount
 	movi	a1, 15
 	wsr	a0, icountlevel

arch/xtensa/platforms/iss/setup.c

@@ -64,7 +64,9 @@ void platform_restart(void)
 			     "wsr	a2, icountlevel\n\t"
 			     "movi	a2, 0\n\t"
 			     "wsr	a2, icount\n\t"
+#if XCHAL_NUM_IBREAK > 0
 			     "wsr	a2, ibreakenable\n\t"
+#endif
 			     "wsr	a2, lcount\n\t"
 			     "movi	a2, 0x1f\n\t"
 			     "wsr	a2, ps\n\t"

arch/xtensa/platforms/xt2000/setup.c

@@ -69,7 +69,9 @@ void platform_restart(void)
 			      "wsr	a2, icountlevel\n\t"
 			      "movi	a2, 0\n\t"
 			      "wsr	a2, icount\n\t"
+#if XCHAL_NUM_IBREAK > 0
 			      "wsr	a2, ibreakenable\n\t"
+#endif
 			      "wsr	a2, lcount\n\t"
 			      "movi	a2, 0x1f\n\t"
 			      "wsr	a2, ps\n\t"

arch/xtensa/platforms/xtfpga/setup.c

@@ -60,7 +60,9 @@ void platform_restart(void)
 			      "wsr	a2, icountlevel\n\t"
 			      "movi	a2, 0\n\t"
 			      "wsr	a2, icount\n\t"
+#if XCHAL_NUM_IBREAK > 0
 			      "wsr	a2, ibreakenable\n\t"
+#endif
 			      "wsr	a2, lcount\n\t"
 			      "movi	a2, 0x1f\n\t"
 			      "wsr	a2, ps\n\t"