armv7: timer: init timer with bootstage

In initf_bootstage() we call bootstage_mark_name() which ends up calling
timer_get_us() before timer_init(); that cause crash for stm32mp1.

This patch solve the issue without changing the initialization sequence.
See also commit 97d20f69 ("Enable CONFIG_TIMER_EARLY with bootstage")
for other solution when DM is activated for TIMER.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>