clk: bcm: rpi: Prevent out-of-bounds access

In the Linux kernel, the following vulnerability has been resolved: clk: bcm: rpi: Prevent out-of-bounds access The while loop in raspberrypi_discover_clocks() relies on the assumption that the id of the last clock element is zero. Because this data comes from the Videocore firmware and it doesn't guarantuee such a behavior this could lead to out-of-bounds access. So fix this by providing a sentinel element.