arm, davinci: add PLL0 prediv to da850 lowlevel setup

The OMAP-L138 has a pre-divider available on PLL0.

Add support to da850_lowlevel.c for configuring PLL0's pre-divider. This is
to achieve certain OPP's -- e.g. the 372MHz OPP used also by Linux.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Cc: Christian Riesch <christian.riesch@omicron.at>
CC: Heiko Schocher <hs@denx.de>
Cc: Sandeep Paulraj <s-paulraj@ti.com>
Cc: Tom Rini <tom.rini@gmail.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Heiko Schocher <hs@denx.de>
Acked-by: Christian Riesch <christian.riesch@omicron.at>