ACPI: introduce acpi_arch_init()

To avoid arch-specific code in general ACPI initialization flow,
introduce a weak symbol acpi_arch_init().

Currently, arm64 and riscv can utillize this to insert their
arch-specific flow.

In the future, other architectures can also have a chance to define
their own arch-specific ACPI initialization process if necessary.

Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Miao Wang <shankerwangmiao@gmail.com>
Link: https://patch.msgid.link/20241121-intro-acpi-arch-init-v4-1-b1fb517e7d8b@gmail.com


[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>