Remove register keywords

The register keyword is deprecated in C++, even at -O0, registers are
used automatically.
Having the register keyword there causes a warning about deprecated
keywords when using clang (and possibly future gcc versions), and
isn't needed.

Change-Id: I0117139d351b737da44177ddada3e03a23763427
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>