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.