mailbox: pcc: Support HW-Reduced Communication Subspace type 2

ACPI 6.1 has a PCC HW-Reduced Communication Subspace type 2 intended for
use on HW-Reduce ACPI Platform, which requires read-modify-write sequence
to acknowledge doorbell interrupt. This patch provides the implementation
for the Communication Subspace Type 2.

Signed-off-by: Hoan Tran <[email protected]>
Reviewed-by: Prashanth Prakash <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>