i2c: designware_i2c: Rewrite timing calculation

At present the driver can end up with timing parameters which are slightly
faster than those expected. It is possible to optimise the parameters to
get the best possible result.

Create a new function to handle the timing calculation. This uses a table
of defaults for each speed mode rather than writing it in code.

The function works by calculating the 'period' of each bit on the bus in
terms of the input clock to the controller (IC_CLK). It makes sure that
the constraints are met and that the different components of that period
add up correctly.

This code was taken from coreboot which has ended up with this same
driver, but now in a much-different form.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>

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