ddr: imx: Handle 3734 in addition to 3733 and 3732 MTps rates

The new MX8M DDR tool 3.31 now generates a programming file which uses
data rate 3734 instead of 3733 or 3732 . Handle another rounding option .

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>