ARM: pxa: add PWM nodes to pxa27x.dtsi

This patch adds PWM nodes for each of the four channels present on the
pxa270.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>