clk: lpc32xx: allow peripheral clock selection in device tree

This patch add the support to select the peripheral clock (PERIPH)
as a parent clock source using the "assigned-clock-parents"
parameter in the device tree.

Signed-off-by: Sylvain Lemieux <slemieux@tycoint.com>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1464982475-24738-1-git-send-email-slemieux.tyco@gmail.com