riscv: efi: Power off via EFI runtime services when available

When booted via UEFI with runtime services enabled, EFI Reset Shutdown is
the firmware-preferred shutdown path: it lets firmware run its own
shutdown hooks which may invoke SBI SRST extension internally. However,
RISC-V always powers off via the SBI SRST extension today and EFI runtime
path is never used even when firmware provides it.

Enable the poweroff via EFI by overriding efi_poweroff_required()

Signed-off-by: Atish Patra <atishp@meta.com>
Reviewed-by: Sunil V L <sunilvl@oss.qualcomm.com>
Link: https://patch.msgid.link/20260615-efi_reset_shutdown-v1-1-9414edcbbab0@meta.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

arch/riscv/kernel/efi.c

@@ -95,3 +95,8 @@ int __init efi_set_mapping_permissions(struct mm_struct *mm,
 				   md->num_pages << EFI_PAGE_SHIFT,
 				   set_permissions, md);
 }
+
+bool efi_poweroff_required(void)
+{
+	return efi_enabled(EFI_RUNTIME_SERVICES);
+}