brcmfmac: allow storing PMU core without wrapper address

Separated PMU core can be found in new devices and should be used for
accessing PMU registers (which were routed through ChipCommon so far).
This core is one of exceptions that doesn't have or need wrapper address
to be still safely accessible.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>