Revert "firmware: raspberrypi: register clk device"

This reverts commit 91f2cf4a.

Revert this because the clock driver won't probe without a DT node, and
creating it as a platform device won't give it one. Doing so avoids the
following error message:

    raspberrypi-clk raspberrypi-clk: Missing firmware node

Signed-off-by: Phil Elwell <phil@raspberrypi.com>