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

mainline inclusion
from mainline-v5.16-rc1
commit f92ae90e
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6Z91B
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v6.3-rc2&id=f92ae90e52bb09d6856ef2785773be59dd633f85



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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>