i2c: stm32f7: allows for any bus frequency

Do not limitate to the 3 (100KHz, 400KHz, 1MHz) bus frequency but
instead allows any frequency (if it matches timing requirements).
Depending on the requested frequency, use the spec data from either
Standard, Fast or Fast Plus mode.

Hardcoding of min/max bus frequencies is removed and is instead computed.

The driver do not use anymore speed identifier but instead handle
directly the frequency and figure out the spec data (necessary
for the computation of the timing register) based on the frequency.

Signed-off-by: Alain Volmat <alain.volmat@st.com>
Reviewed-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>