pinctrl: sunxi: Rename wemac functions to emac

The pin functions associated to the ethernet controller were introduced
with the label "wemac", which was the name of the ethernet driver in the
allwinner source tree, while the real name of the IP is actually "emac",
as advertised in several documents including the datasheet.

Since this is part of the device tree bindings, and that these part of
the bindings have not yet be released in a kernel version, it seems like
the good timing to change it and assure consistency.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>