apm821xx: set DEVICE_TYPE to "nas" for sata subtarget

apm821xx: set DEVICE_TYPE to "nas" for sata subtarget

Since DEVICE_TYPE cannot be set per device, just set DEVICE_TYPE
to "nas" for the entire subtarget, which only contains this single
device.

Note that while this looks like a cosmetic change in combination
with the previous patches, this particular patch actually changes
the packages for the device.

Suggested-by: Christian Lamparter <chunkeey@gmail.com>
Cc: Christian Lamparter <chunkeey@gmail.com>
Cc: Sungbo Eo <mans0n@gorani.run>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>