i2c: stm32_v2: implement slave support

This patch adds I2C Slave support conforming to the syscalls and funcs,
only for the STM32 V2 I2C Driver for the moment.

It is capable of handling multi-master bus setups.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Wagenknecht <wagenknecht@clage.de>