dt-bindings: clk: stm32h7: fix clock-cell size

The clock-cell size is 1 on stm32h7 plaform.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Fixes: 3e4d618b ("clk: stm32h7: Add stm32h743 clock driver")
Signed-off-by: Rob Herring <robh@kernel.org>