sunxi: Fix CONFIG_SUNXI_GMAC references

SUNXI_GMAC was still used to configure the code where as the
same has been renamed and moved to Kconfig in below commit
"sunxi: Move SUNXI_GMAC to Kconfig"
(sha1: 4d43d065)

Signed-off-by: Dave Prue <dave@prue.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Tested-by: Mark Kettenis <kettenis@openbsd.org>
[Tweek commit message, config_whitelist.txt, build-whitelist.sh]
Signed-off-by: Jagan Teki <jagan@openedev.com>