nvme: Enable FUA

Most NVME devcies maintain data in internal cache for an uncertain
times, and u-boot has no method to force NVME to flush cache.
So this patch adds FUA to avoid data loss caused by power off after data
programming.

Signed-off-by: Jon Lin <jon.lin@rock-chips.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>