clk: socfpga: Fix integer overflow in clock calculation

Use 64-bit integer for calculating clock rate. Also use do_div for the
64-bit division.

Signed-off-by: Graham Moore <grmoore@altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Mike Turquette <mturquette@linaro.org>