hw/intc/sifive_plic: Take a MemoryRegion for the created device

sifive_plic_create() maps the device it creates into system_memory,
which prevents its use by SoCs that map devices into their own memory
container.

Add a MemoryRegion parameter and map the device into it instead of
calling sysbus_mmio_map(). All callers pass system_memory, so there is
no change in behaviour.

Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Chao Liu <chao.liu@processmission.com>
Message-ID: <20260903055131.257903-6-joel@jms.id.au>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>