i2c: tegra: Add de-bounce cycles.

This enables debouncing of the I2C lines. The debounce period is
2 * the debounce register field value, in terms of the I2C block's main
clock. The Tegra TRM indicates that a setting yielding >50nS is
desirable. Hence, a setting of 2 => 4 clocks @ 72MHz => ~55nS.

Signed-off-by: Ken Radtke <kradtke@nvidia.com>
[swarren: Added commit description body,
 Fixed 80-column limit, Reverted file permission change]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>