Generalizes dm644x pinmux.

The existing pinmux layer works only when the PINMUX register has single bit
field to enable the secondary function. DM646x can support secondary as well
as tertiary pin functions. This new pinmux layer is similar to the one being
used by OMAP architecture.

Checkpatch script reports a false positive error at line no.465 of this patch.

This patch is based on the work originally done by Vladimir Barinov, when he
was in MontaVista.
Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>