clocksource: convert SPEAr platforms 16-bit up counting clocksource

Convert SPEAr platforms 16-bit up counting clocksource, which requires
a 16-bit register access read rather than 32-bit.

Acked-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index bab25a0..cbc27fb 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -884,6 +884,7 @@
 	select ARM_AMBA
 	select ARCH_REQUIRE_GPIOLIB
 	select CLKDEV_LOOKUP
+	select CLKSRC_MMIO
 	select GENERIC_CLOCKEVENTS
 	select HAVE_CLK
 	help