i2c: stm32f7: SYSCFG Fast Mode Plus support for I2C STM32F7

Read SYSCFG bindings to set Fast Mode Plus bits if Fast Mode Plus
speed is selected.

Handle the stm32mp15 specific compatible to handle FastMode+
registers handling which is different on the stm32mp15 compared
to the stm32f7 or stm32h7.
Indeed, on the stm32mp15, the FastMode+ set and clear registers
are separated while on the other platforms (F7 or H7) the control
is done in a unique register.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Heiko Schocher <hs@denx.de>