mmc: sdhci_am654: Add support for input tap delay

mmc: sdhci_am654: Add support for input tap delay

DLL need only be enabled for speed modes and clock frequencies at or
above 50 MHz. For speed modes that don't enable the DLL, we need to
configure a static input delay value. This involves reading an optional
itap-del-sel-* value from the device tree and configuring it for the
appropriate speed mode.

With this addition, make sure that DLL is always switched off at the
beginning of the set_clock() call to simplify configuration. This also
removes the need for the dll_on member in struct sdhci_am654_data.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Link: https://lore.kernel.org/r/20200923105206.7988-5-faiz_abbas@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>