PCI: brcmstb: Accommodate MSI for older chips

Older BrcmSTB chips do not have a separate register for MSI interrupts; the
MSIs are in a register that also contains unrelated interrupts.  In
addition, the interrupts lie in bits [31..24] for these legacy chips.  This
commit provides common code for both legacy and non-legacy MSI interrupt
registers.

Link: https://lore.kernel.org/r/20200911175232.19016-9-james.quinlan@broadcom.com


Signed-off-by: Jim Quinlan <jquinlan@broadcom.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>