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

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

Commit ceda2fb9f36d ("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: ceda2fb9f36d ("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>