tegra: Correct logic for reading pll_misc in clock_start_pll()

The logic for simple PLLs on T124 was broken by this commit:

  722e000c Tegra: PLL: use per-SoC pllinfo table instead of PLL_DIVM/N/P, etc.

Correct it by reading from the same pll_misc register that it writes to and
adding an entry for the DP PLL in the pllinfo table.

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