arm/tegra: prepare Seaboard pinmux code for derived boards

This patch splits out the common part of pinmux and GPIO initialization for
seaboard and derived boards. This code is based on work done by Jong Kim
<jongk@nvidia.com>.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>