clk: tegra: Fix pll_u rate configuration

Turns out latest upstream U-Boot does not configure/enable pll_u which
leaves it at some default rate of 500 kHz:

root@apalis-t30:~# cat /sys/kernel/debug/clk/clk_summary | grep pll_u
       pll_u                  3        3        0      500000          0

Of course this won't quite work leading to the following messages:

[    6.559593] usb 2-1: new full-speed USB device number 2 using tegra-
ehci
[   11.759173] usb 2-1: device descriptor read/64, error -110
[   27.119453] usb 2-1: device descriptor read/64, error -110
[   27.389217] usb 2-1: new full-speed USB device number 3 using tegra-
ehci
[   32.559454] usb 2-1: device descriptor read/64, error -110
[   47.929777] usb 2-1: device descriptor read/64, error -110
[   48.049658] usb usb2-port1: attempt power cycle
[   48.759475] usb 2-1: new full-speed USB device number 4 using tegra-
ehci
[   59.349457] usb 2-1: device not accepting address 4, error -110
[   59.509449] usb 2-1: new full-speed USB device number 5 using tegra-
ehci
[   70.069457] usb 2-1: device not accepting address 5, error -110
[   70.079721] usb usb2-port1: unable to enumerate USB device

Fix this by actually allowing the rate also being set from within
the Linux kernel.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>