bcma: support PMU present as separated bus core

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.:
[    0.000577] Unhandled fault: external abort on non-linefetch (0x1008) at 0xf1000604

Solve it by using a new (PMU) core pointer set to ChipCommon or PMU
depending on the hardware capabilities.

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