gpio/tegra: Convert to a platform device

v3: Make regs variable static. Remove empty init of tegra_gpio_banks.

v2: Retrieve IRQ and memory addresses from resources instead of hard-
coding them. Add back initialization of tegra_gpio_chip.of_node.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
[olof: switched probe routine to __devinit]
Signed-off-by: Olof Johansson <olof@lixom.net>