net: ethernet: mtk-star-emac: select REGMAP_MMIO

The driver depends on mmio regmap API but doesn't select the appropriate
Kconfig option. This fixes it.

Fixes: 8c7bd5a4 ("net: ethernet: mtk-star-emac: new driver")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20201020073515.22769-1-brgl@bgdev.plSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mediatek/Kconfig

@@ -17,6 +17,7 @@ config NET_MEDIATEK_SOC
 config NET_MEDIATEK_STAR_EMAC
 	tristate "MediaTek STAR Ethernet MAC support"
 	select PHYLIB
+	select REGMAP_MMIO
 	help
 	  This driver supports the ethernet MAC IP first used on
 	  MediaTek MT85** SoCs.