mailbox: pcc: Add PCC register bundle and associated accessor functions

Extended PCC subspaces introduces more registers into the PCCT. In order
to consolidate access to these registers and to keep all the details
contained in one place, let us introduce PCC register bundle that holds
the ACPI Generic Address Structure as well as the virtual address for
the same if it is mapped in the OS.

It also contains the various masks used to access the register and
the associated read, write and read-modify-write accessors.

We can also clean up the initialisations by having a helper function
for the same.

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