pinctrl: dra: dt-bindings: Add virtual mode configuration option

In addition to the regular mux configuration such as mux mode 1,
2 etc, 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.

It is important to note that the official TI guidelines recommend
to do as minimal pin reconfiguration beyond the bootloader given
the design of the hardware involved which can result in substantial
glitches which may impair functionality of certain peripherals.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>