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>