clk: stm32mp: change fdt_get_rcc_node as static

Change fdt_get_rcc_node as static, cometic change,
no functionnal change.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: Ib4ef110f6f1b16dbaa727a065e40275d3cf58a73
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/180867


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>