meta-phosphor: jsnbd: promote dynamic-layers content

The previous 'dynamic-layers' content has been fixed so that it is
functional on both aspeed and nuvoton (and likely any system that
exposes the same sysfs class entries).  Promote this content out
of dynamic-layers and make it common to reduce the hardware-specific
duplication.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8f8247098ef8f8b035269815ebe77dedb2e8a441