arm: use common instructions applicable to armv7m & other arm archs

This patch cleans the code by using instructions allowed for armv7m as well as
other Arm archs.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>