Convert CONFIG_RGMII to Kconfig. Thanks to that, it is possible to
update defconfig files of SYS_EXTRA_OPTIONS accordingly and
remove it when it is possible.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>