davinci: split out some dm644x-specific bits from psc

Split out DaVinci DM6446-specific bits from more generic bits:

 - Add a CONFIG_SOC_DM644X.  All current boards use DM6446 chips;
   DM6443 and DM6441 chips differ in available peripherals.

 - Move most DM644X-specific bits from psc.c to a new dm644x.c file,
   which is conditionally built.  It provides device-specific setup.

Plus minor coding style and comment updates with respect to the PSC.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
diff --git a/cpu/arm926ejs/davinci/Makefile b/cpu/arm926ejs/davinci/Makefile
index e09874b..6052e02 100644
--- a/cpu/arm926ejs/davinci/Makefile
+++ b/cpu/arm926ejs/davinci/Makefile
@@ -28,6 +28,7 @@
 LIB	= $(obj)lib$(SOC).a
 
 COBJS-y				+= timer.o psc.o
+COBJS-$(CONFIG_SOC_DM644X)	+= dm644x.o
 COBJS-$(CONFIG_DRIVER_TI_EMAC)	+= ether.o lxt972.o dp83848.o
 
 SOBJS	= reset.o