arch: arm: Use the armv8_timer.h for the Cortex A7

We can't use the timer.h as the arm timer
is within the soc and does not have an address.

Instead we use the armv8_timer.h, renaming it to armv7_v8_timer.h
for the Cortex-A7 as the cp15 access are compatible.

Signed-off-by: Julien Racki <julien.racki@st.com>