gpio: tegra: Parameterize the number of banks

Tegra20's GPIO controller has 7 banks, and Tegra30's controller has 8
banks. Allow the number of banks to be configured at run-time by the
device tree.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Olof Johansson <olof@lixom.net>