brcmfmac: access PMU registers using standalone PMU core if available

On recent Broadcom chipsets PMU is present as separated core and it
can't be accessed using ChipCommon anymore as it fails with e.g.:
[   18.198412] Unhandled fault: imprecise external abort (0x1406) at 0xb6da200f

Add a new helper function that will return a proper core that should be
used for accessing PMU registers.

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