ARM: dts: omap4-panda: move generic sections to panda-common

PandaBoard, PandaBoard-A4 revisions use OMAP4430.
PandaBoard-ES version of the board uses OMAP4460.

Move the original panda dts file into a common dtsi used by all panda
variants. This allows us to introduce SoC variation for PandaBoard ES
without impacting other PandaBoard versions that are supported.
As part of this change, since OMAP4460 adds on to OMAP4430, add
omap4.dtsi to omap4460.dtsi.

Signed-off-by: Nishanth Menon <nm@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Jon Hunter <jon-hunter@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Keerthy <j-keerthy@ti.com>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>