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.