ARM: dts: Add build target for omap4-panda-a4

Commit 0d9250c (ARM: dts: omap4-panda: Add pinmux configuration for
HDMI) added a new device-tree source file for Rev A4 of the OMAP4430
Panda board but it did not add this version to the makefile. Hence,
add this file to the makefile.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>