mmc: sunxi: sun4i / sun5i do not have sample clocks

It turns out that sun4i (A10) and sun5i (A13 & co) do not have sample
clocks, so add a new sun7i-a20-mmc compatible and do not try to use
sample clocks on sun4i / sun5i.

Since sun4i / sun5i do not have sample clocks, they cannot (reliably) do
DDR rates, so only set MMC_CAP_1_8V_DDR when we do have sample clks.

Note this patch leaves the clk_prepare_enable() / clk_disable_unprepare()
calls to the sample clks as-is, without adding checks for them being
NULL. All the clk_foo calls accept a NULL clk and will return success when
called with a NULL clk.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>