clk: Add directory for STM32 clock drivers

Add a directory in drivers/clk to regroup the clock drivers for all
STM32 Soc with CONFIG_ARCH_STM32 (MCUs with cortex M) or
CONFIG_ARCH_STM32MP (MPUs with cortex A).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Grzegorz Szymaszek <gszymaszek@short.pl>
Acked-by: Sean Anderson <seanga2@gmail.com>
Change-Id: I955af307963f732167396f0157a30cf2fc91f150