arm/tegra: Rework Seaboard-vs-Ventana pinmux table

Rather than modifying seaboard_pinmux[] using ventana_pinmux[], split
seaboard_pinmux[] into common_pinmux[] and seaboard_pinmux[], and then
actually apply common_pinmux[] always, followed by the appopriate one
of seaboard_pinmux[] or ventana_pinmux[].

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>