arm64 / ACPI: clean the additional checks before calling ghes_notify_sea()

In order to remove the additional check before calling the
ghes_notify_sea(), make stub definition when !CONFIG_ACPI_APEI_SEA.

After this cleanup, we can simply call the ghes_notify_sea() to let
APEI driver handle the SEA notification.

Signed-off-by: Dongjiu Geng <gengdongjiu@huawei.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>