ARM: stm32: restore reboot capabilities

To be able to reboot the platform we need to use armv7m_restart for STM32
SoCs using ARMv7 cores.

Fixes: e0644101bd2f ("ARM: mach-stm32: prepare stm32 family to welcome armv7 architecture)

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>