ARM: tegra: clock: add clk_id_to_pll_id helper

This function allows to convert a device tree clock ID to PLL ID.

Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # ASUS TF T30
Tested-by: Robert Eckelmann <longnoserob@gmail.com> # ASUS TF101 T20
Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # HTC One X
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Signed-off-by: Tom <twarren@nvidia.com>