allwinner/a83t_padconf: Rename "emac" function to "gmac" as per upstream DTS

Although these should have been 'emac', upstream DTS is going with using
'gmac' as the function name for the emac RGMII pins. Rename here to
accommodate.

emac support for the a83t should come in with the 4.16 DTS update, in
another couple of months.