MAINTAINERS: Add Alain Volmat as STM32 I2C/SMBUS maintainer

Add Alain Volmat as STM32 I2C/SMBUS driver co-maintainer.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: alain.volmat@foss.st.com
Link: https://lore.kernel.org/r/20210216074929.29033-4-patrice.chotard@foss.st.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>