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.