nvme: add accessor to namespace id and eui64

nvme: add accessor to namespace id and eui64

This adds a function which can be used by e.g. EFI to retrieve
the namespace identifier and EUI64.  For that it adds the EUI64
to its driver internal namespace structure and copies the EUI64
during namespace identification.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>