ARM: tegra: clock: add clock_decode_pair helper

Get periph clock id and its parent from device tree.
This works by looking up the peripheral's 'clocks' node and
reading out the second and fourth cells, which are the
peripheral and PLL clock numbers.

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>