ARM: dts: omap4: add twd clock to DT

Booting Linux 3.14 on Pandaboard currently gets the following
message displayed:

smp_twd: clock not found -2

Define "mpu_periphclk" as the twd clock in omap4 dts to avoid this.

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>