clk: stm32mp1: correct CKSELR masks

Correct three masks used to access on the RCC register
RCC_QSPICKSELR, RCC_FMCCKSELR and RCC_ADCCKSELR: only 3 bits.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>