Blackfin: add support for gptimer0 as a tick source

For systems where the core cycles are not a usable tick source (like SMP
or cycles gets updated), enable gptimer0 as an alternative.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/arch/blackfin/mach-bf537/Kconfig b/arch/blackfin/mach-bf537/Kconfig
index bbc08fd..d81224f 100644
--- a/arch/blackfin/mach-bf537/Kconfig
+++ b/arch/blackfin/mach-bf537/Kconfig
@@ -66,6 +66,7 @@
 	default 11
 config IRQ_TIMER0
 	int "IRQ_TIMER0"
+	default 7 if TICKSOURCE_GPTMR0
 	default 8
 config IRQ_TIMER1
 	int "IRQ_TIMER1"