clk: tegra: Fix PLL_U post divider and initial rate on Tegra30

The post divider value in the frequency table is wrong as it would lead
to the PLL producing an output rate of 960 MHz instead of the desired
480 MHz. This wasn't a problem as nothing used the table to actually
initialize the PLL rate, but the bootloader configuration was used
unaltered.

If the bootloader does not set up the PLL it will fail to come when used
under Linux. To fix this don't rely on the bootloader, but set the
correct rate in the clock driver.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>