clk: stm32mp1: fix CK_MPU calculation

When the CK_MPU used PLL1_MPUDIV, the current rate is
wrong. The clock must use stm32mp1_mpu_div as a shift
value. Fix the check value used to enter PLL_MPUDIV.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>