clk: st: flexgen: remove unused compatible

Following B2120 boards removal in commit dee546e1 ("ARM: sti: drop
B2120 board support"), several compatibles are left unused.  Remove
them.

Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Brian Masney <bmasney@redhat.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>