Davinci: eliminate pinmux offset verbosity

Pinmux registers are sequential, and do not need to be enumerated out as they
currently are.  This reduces code volume and keeps things simple.

If some future SoC comes up with a discontiguous register map, PINMUX() can
then be expanded with local token pasting.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>