build: Add targets for auto gen of asm-offsets.h and use it in imx35

asm-offsets.h should be auto generated. This patch adds two rules to rules.mk
which makes this possible and removes the rules on imx35.

Signed-off-by: Matthias Weisser <weisserm@arcor.de>
Acked-by: Stefano Babic <sbabic@denx.de>