ARM: dts: omap4/5: Use l3_ick for the gpmc node

The GPMC clock is derived from l3_ick. The simplest solution is
to reference directly l3_ick to provide the GPMC fck in order to
get correct timings. The real management of the clock is left to
hwmod.

Cc: stable@vger.kernel.org # v3.14+
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Tony Lindgren <tony@atomide.com>