clk: stm32mp: preserved PLL4 for USB boot

The PLL4 can be use by ROM code as source clock of USB PHYC
In this case the PLL4 configuration must be preserved.

This patch add a error when the PLL4 configuration change
during the clock tree initialization.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: I4bae9312a2db8dd342a38e649513d689b13976bb
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/177379


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>
Tested-by: Yann GAUTIER <yann.gautier@st.com>