soc: bcm: Check for NULL return of devm_kzalloc()

In the Linux kernel, the following vulnerability has been resolved: soc: bcm: Check for NULL return of devm_kzalloc() As the potential failure of allocation, devm_kzalloc() may return NULL. Then the 'pd->pmb' and the follow lines of code may bring null pointer dereference. Therefore, it is better to check the return value of devm_kzalloc() to avoid this confusion.