mmc: sunxi: Cleanup "new timing mode" selection

Among the SoCs using the "new timing mode", only the A83T needs to
explicitly switch to that mode.

By just defining the symbol for that one odd A83T bit to 0 for any other
SoCs, we can always OR that in, and save the confusing nested #ifdefs.

Clean up the also confusing new_mode setting on the way.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>