ARM: 6722/1: SPEAr: sp810: switch to slow mode before reset

In sysctl_soft_reset(), switch to slow mode before resetting the system
via the system controller.  This is required.

Reviewed-by: Stanley Miao <stanley.miao@windriver.com>
Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>