tegra2: Rename CLOCK_PLL_ID to CLOCK_ID

Rename CLOCK_PLL_ID to CLOCK_ID which takes account of the fact that the
code now deals with both PLL clocks and source clocks.

This also tidied up the assert() to match the one sent upstream, and fixes
an error in the PWM id.

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