OMAP2/3: GPMC: put sync_clk value in picoseconds instead of nanoseconds

The calculations done with sync_clk are anyway in picoseconds
and switching to picoseconds allows sync_clk values that are
not a whole number of nanoseconds - which is sometimes the
case.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>