watchdog: sp805: replace readl/writel with lighter _relaxed variants

readl/writel versions for ARM contain memory barrier instruction for
synchronizing DMA buffers.  These are not required at least on this
module.  So use lighter _relaxed variants.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>