mmc: sunxi: Replace H6 ifdefs with H6 gen macro

It turns out that several SoCs share same mmc configuration as H6. In
order to lower ifdef clutter replace H6 specific macro with common one.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>