mailbox: pcc: Consolidate subspace doorbell register parsing

mainline inclusion
from mainline-v5.16-rc1
commit 4e3c96ff
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=4e3c96ff950ed2bf0f8ef24bd54ec134e2717c55



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Extended PCC subspaces(Type 3 and 4) differ from generic(Type 0) and
HW-Reduced Communication(Type 1 and 2) subspace structures. However some
fields share same offsets and same type of structure can be use to
extract the fields. In order to simplify that, let us move all the doorbell
register parsing into pcc_parse_subspace_db_reg and consolidate there.
It will be easier to extend it if required within the same.

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