ARM: mx25: rename mach-mx25pdk.c to mach-mx25_3ds.c matching its arch number

Additionally document all known names of that machine.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/arm/mach-mx25/Makefile

 obj-y				:= mm.o devices.o
 obj-$(CONFIG_ARCH_MX25)		+= clock.o
-obj-$(CONFIG_MACH_MX25_3DS)	+= mach-mx25pdk.o
+obj-$(CONFIG_MACH_MX25_3DS)	+= mach-mx25_3ds.o

arch/arm/mach-mx25/mach-mx25pdk.c → arch/arm/mach-mx25/mach-mx25_3ds.c

@@ -16,6 +16,12 @@
  * Boston, MA  02110-1301, USA.
  */
 
+/*
+ * This machine is known as:
+ *  - i.MX25 3-Stack Development System
+ *  - i.MX25 Platform Development Kit (i.MX25 PDK)
+ */
+
 #include <linux/types.h>
 #include <linux/init.h>
 #include <linux/delay.h>