Standarize Allwinner EMAC name

The linux driver name is called 'wemac', but in the datasheet calls it
'emac'. Not only that, A20 calls it gmac. Other chips also use 'emac'
for their ethernet mac's. The W seems like it was a typo that stuck.
Rename wemac to emac

Rename wemac to emac as described in the previous patch