stm32mp1: update I2C and PMIC drivers

Regulator configuration at boot takes more information from DT.
I2C configuration from DT is done in I2C driver.
I2C driver manages more transfer modes.
The min voltage of buck1 should also be increased to 1.2V,
else the platform does not boot.

Heavily modifies stm32_i2c.c since many functions move inside the source
file to remove redundant declarations.

Change-Id: I0bee5d776cf3ff15e687427cd6abc06ab237d025
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Nicolas LE BAYON <nicolas.le.bayon@st.com>