The AR100 clock within the R_CCU (PRCM) has the PLL_PERIPH0 as one of
its parents.

This adds the reference in the device tree describing this relationship.
This patch uses a raw number for the clock index to ease merging by
avoiding cross tree dependencies.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>