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.

Fixes: 8bcac401 ("soc: bcm: add PM driver for Broadcom's PMB")
Signed-off-by: QintaoShen <unSimple1993@163.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>