This is a re-application of fix 17644b55.
arm_cpu_lowlevel_init() will set the processor mode to 0x13 (supervisor).
When this function is entered via a different processor mode, register
banking will happen to lr (r14), resulting in an invalid return address.
This fix will preserve the return address manually.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>