clk: clk_stm32f7: fix PLL clock division factor

Fix clock division factor initialization for RCC_PLLCFGR
registers.

PLLR bits (bit 31-28) in RCC_PLLCFGR must not be cleared,
it's a forbidden value. So update RCC_PLLCFGR using
clrsetbits_le32() to set only necessary bits fields.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>