ARM: 7348/1: arm/spear600: fix one-shot timer

Currently, the "clockevent_next_event" function only works correctly
if the timer is not running when this function is called, which is
not always the case when running with CONFIG_HIGH_RES_TIMERS.

Fix this by stopping the timer at the beginning of this function.

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Acked-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>