ARM: DRA7: Add support for virtual mode configuration

In addition to the regular mux configuration, certain pins of DRA7
require to have "virtual mode" also programmed.
This allows for predefined delay characteristics to be used by the SoC
to meet timing characterstics needed for the interface.

Provide easy to use macro to do the same.

For configuring virtual mode, along with normal pad configuration add
the following two steps:
- Select MODESELECT field of each assocaited PAD.
  CTRL_CORE_PAD_XXX[8]:MODESELECT = 1
- DELAYMODE filed should be configured with value given in DATA Manual.
  CTRL_CORE_PAD_XXX[7:4]:DELAYMODE =[0-15] (as given in DATA manual).

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>