mmc: sunxi: Clarify new timing mode usage and implementation

Newer sunxi mmc controller variants support what they call the "new
timing mode". Support for this was implemented in two ways, according
to the hardware that was seen at the time.

The first type retained the old timing mode, and both the clock and mmc
controllers had switches to select which mode was used. Both switches
had to be set to the same setting. This variant was denoted with the
.has_timings_switch field in the sunxi_mmc_cfg structure. This hardware
is only seen on the A83T.

The second type did away with the old timing mode. The clock controller
no longer had the mode selection or clock delay setting bits. In some
cases the mmc controller retained its mode selection bit, but this
always needed to be set to the new mode, or instabilities would occur.
In a few cases, such as the A64 and H6 eMMC controller, the mode
selection bit is gone, but the controller still behaves like the new
timing mode, requiring the module clock to be double the card clock
in DDR transfer modes. This variant is denoted with the
.needs_new_timings field.

This patch adds more comments explaining the two fields, as well as
the possibly nonexistent mode switch in the mmc controller.

The .has_timings_switch is renamed to .ccu_has_timings_switch to clarify
its meaning.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>