mmc: mtk-sd: Add HS400 online tuning support

According to JEDEC Spec, there is no need to do tuning under HS400 mode
since the Rx signal is aligned with the DS signal. However, MediaTek's IC
need set its "DS delay" internally to ensure it can latch Rx signal
correctly.

In previous version, We provide an "hs400-ds-delay" in device tree to cover
different chipset/PCB design, and it works fine in most cases.  But, with
the development of process technology and the big VCore voltage scale
range(may have 0.7V/0.6V/0.55V), it is difficult to find a suitable
"hs400-ds-delay" to cover all of IC corner cases(SSSS/TTTT/FFFF). So that
We must have the ability to do hs400 online tuning.

Signed-off-by: Wenbin Mei <wenbin.mei@mediatek.com>
Reviewed-by: Chaotian Jing <chaotian.jing@mediatek.com>
Link: https://lore.kernel.org/r/20210917124803.22871-4-wenbin.mei@mediatek.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>