clocksource/drivers/h8300_timer8: Separate the Kconfig option from the arch

The current Kconfig option is the H8300 arch option. In order to comply to the
current rule, let's create a specific option for the timer8 and select it
from the arch's Kconfig.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index de331b8..73477b5 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig
@@ -362,6 +362,9 @@
 	  This enables OST0 support available on PXA and SA-11x0
 	  platforms.
 
+config H8300_TMR8
+        bool
+
 config H8300_TMR16
         bool