clk: stm32mp_clkfunc: keep RCC node offset in a static variable

To avoid parsing device tree file too often, keep the RCC node offset
value in a variable in fdt_get_rcc_node().

Change-Id: Ibb23ff92247d57c65a23517b8f3473f639794d2a
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/181775


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>