arm, davinci: Add lowlevel_init for SoCs other than DM644X

The low level initialization code in
arch/arm/cpu/arm926ejs/davinci/lowlevel_init.S was written for
DM644X SoCs only. This patch makes the lowlevel_init function in this
file a dummy function for SoCs other than DM644X.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Tom Rini <trini@ti.com>
Cc: Sergey Kubushyn <ksi@koi8.net>
Acked-by: Heiko Schocher <hs@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>