mmc: meson-gx: add basic tuning for rx clock phase

This patch adds basic tuning which changes the rx clock phase only
until a working setting is found.

On a Odroid C2 with 128GB eMMC card and 200 MHz MMC clock only
180° rx clock phase make the system boot w/o CRC errors.

With other MMC devices / clock speeds this might be different,
therefore don't change the driver config in general.

When retuning skip the currently active parameter set. This avoids
the current problematic config to be chosen again if it causes CRC
errors just occasionally.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>