dt-bindings: net: Remove duplicate NSP Ethernet MAC binding document

Commit 07d4510f ("dt-bindings: net: bgmac: add bindings documentation for
bgmac") added both brcm,amac-nsp.txt and brcm,bgmac-nsp.txt. The former is
actually the one that got updated and is in use by the bgmac driver while the
latter is duplicating the former and is not used nor updated.

Fixes: 07d4510f

 ("dt-bindings: net: bgmac: add bindings documentation for bgmac")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>