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>