arm: factorize relocate_code routine

Replace all relocate_code routines from ARM start.S files
with a single instance in file arch/arm/lib/relocate.S.
For PXA, this requires moving the dcache unlocking code
from within relocate_code into c_runtime_cpu_setup.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Tested-by: Simon Glass <sjg@chromium.org>