clk: stm32mp1_clk: use correct return value

The function stm32mp1_clk_init() returns an int. Return an error value
if the device tree is not found.

Change-Id: I422d5fea46c4d63d55a5b62e1db154c1f53f41b7
Signed-off-by: Yann Gautier <yann.gautier@st.com>