ppc4xx: Clarify comment about boot chip-select in start.S

Ths old comment was quite screwed up. Replace it with a new version
that should be a bit more descriptive.

Signed-off-by: Stefan Roese <sr@denx.de>

arch/powerpc/cpu/ppc4xx/start.S

@@ -48,21 +48,23 @@
  *-------------------------------------------------------------------------------
  */
 
-/*  U-Boot - Startup Code for AMCC 4xx PowerPC based Embedded Boards
- *
+/*
+ * Startup code for IBM/AMCC PowerPC 4xx (PPC4xx) based boards
  *
- *  The processor starts at 0xfffffffc and the code is executed
- *  from flash/rom.
- *  in memory, but as long we don't jump around before relocating.
- *  board_init lies at a quite high address and when the cpu has
- *  jumped there, everything is ok.
- *  This works because the cpu gives the FLASH (CS0) the whole
- *  address space at startup, and board_init lies as a echo of
- *  the flash somewhere up there in the memorymap.
+ * The following description only applies to the NOR flash style booting.
+ * NAND booting is different. For more details about NAND booting on 4xx
+ * take a look at doc/README.nand-boot-ppc440.
  *
- *  board_init will change CS0 to be positioned at the correct
- *  address and (s)dram will be positioned at address 0
+ * The CPU starts at address 0xfffffffc (last word in the address space).
+ * The U-Boot image therefore has to be located in the "upper" area of the
+ * flash (e.g. 512MiB - 0xfff80000 ... 0xffffffff). The default value for
+ * the boot chip-select (CS0) is quite big and covers this area. On the
+ * 405EX this is for example 0xffe00000 ... 0xffffffff. U-Boot will
+ * reconfigure this CS0 (and other chip-selects as well when configured
+ * this way) in the boot process to the "correct" values matching the
+ * board layout.
  */
+
 #include <asm-offsets.h>
 #include <config.h>
 #include <asm/ppc4xx.h>