mailbox: pcc: Use acpi_os_ioremap() instead of ioremap()

The Platform Communication Channel (PCC) mailbox driver currently uses
ioremap() to map channel shared memory regions. However it is preferred
to use acpi_os_ioremap(), which is mapping function specific to EFI/ACPI
defined memory regions. It ensures that the correct memory attributes
are applied when mapping ACPI-provided regions.

While at it, also add checks for handling any errors with the mapping.

Acked-by: Huisong Li <lihuisong@huawei.com>
Tested-by: Huisong Li <lihuisong@huawei.com>
Tested-by: Adam Young <admiyo@os.amperecomputing.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>