Documentation: dt: add bindings for keystone pll control controller

Documentation: dt: add bindings for keystone pll control controller

The main pll controller used to drive theC66x CorePacs, the switch fabric,
and a majority of the peripheral clocks (all but the ARM CorePacs, DDR3 and
the NETCP modules) requires a PLL Controller to manage the various clock
divisions, gating, and synchronization.

Reviewed-by: Arnd Bergmann <[email protected]>
Acked-by: Mike Turquette <[email protected]>
Signed-off-by: Ivan Khoronzhuk <[email protected]>
[[email protected]: Fixed the subject line]
Signed-off-by: Santosh Shilimkar <[email protected]>