tegra: clock: Add checking for invalid clock IDs

The get_pll() function can do the wrong thing if passed values that are
out of range. Add checks for this and add a function which can return
a 'simple' PLL. This can be defined by SoCs with their own clocks.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>