clk: bcm: rpi: Add NULL check in raspberrypi_clk_register()

In the Linux kernel, the following vulnerability has been resolved: clk: bcm: rpi: Add NULL check in raspberrypi_clk_register() devm_kasprintf() returns NULL when memory allocation fails. Currently, raspberrypi_clk_register() does not check for this case, which results in a NULL pointer dereference. Add NULL check after devm_kasprintf() to prevent this issue.