refactor(stm32mp_clk): keep RCC node offset
refactor(stm32mp_clk): keep RCC node offset

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 <e753f6969c288905606e3c07464886d4f1fc1be0@st.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware