mmc: am654_sdhci: 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.

Therefore, move all dll configurations to their own functions and gate it
with 50 MHz speed and a minimum mode. If both these conditions are not
satisfied then configure delay chain modes.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>