spi: tegra20-slink: Add OPP support

The SPI on Tegra belongs to the core power domain and we're going to
enable GENPD support for the core domain. Now SPI driver must use OPP
API for driving the controller's clock rate because OPP API takes care
of reconfiguring the domain's performance state in accordance to the
rate. Add OPP support to the driver.

Acked-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>