Fix inline assembly for clang

add r8, r0, asl #2
and
add r8, r8, r0, asl #2

are identical in gcc inline assembly, but clang accepts only the latter
variant.

Change-Id: I922367984b6ae2f7268ce99facaf8da425c39d7e
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>