soc: hisilicon: kunpeng_hccs: Simplify PCC shared memory region handling

The PCC driver now handles mapping and unmapping of shared memory
areas as part of pcc_mbox_{request,free}_channel(). Without these before,
this Kunpeng HCCS driver did handling of those mappings like several
other PCC mailbox client drivers.

There were redundant operations, leading to unnecessary code. Maintaining
the consistency across these driver was harder due to scattered handling
of shmem.

Just use the mapped shmem and remove all redundant operations from this
driver.

Cc: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>