mailbox: pcc: Rename doorbell ack to platform interrupt ack register

The specification refers this register and associated bitmask as platform
interrupt acknowledge register. Let us rename it so that it is easier to
map and understand.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>