doc: Remove obsolete doc/README.nand-boot-ppc440

The PPC440 support has been removed in commit 98f705c9
("powerpc: remove 4xx support"). This patch removes this obsolete
file as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>

doc/README.nand-boot-ppc440

------------------------------
-NAND boot on PPC440 platforms
------------------------------
-
-This document describes the U-Boot NAND boot feature as it
-is implemented for the AMCC Sequoia (PPC440EPx) board.
-
-The PPC440EP(x)/GR(x) cpu's can boot directly from NAND FLASH,
-completely without NOR FLASH. This can be done by using the NAND
-boot feature of the 440 NAND flash controller (NDFC).
-
-Here a short description of the different boot stages:
-
-a) IPL (Initial Program Loader, integrated inside CPU)
-------------------------------------------------------
-Will load first 4k from NAND (SPL) into cache and execute it from there.
-
-b) SPL (Secondary Program Loader)
----------------------------------
-Will load special U-Boot version (NUB) from NAND and execute it. This SPL
-has to fit into 4kByte. It sets up the CPU and configures the SDRAM
-controller and the NAND controller so that the special U-Boot image can be
-loaded from NAND to SDRAM.
-This special image is build in the directory "nand_spl".
-
-c) NUB (NAND U-Boot)
---------------------
-This NAND U-Boot (NUB) is a special U-Boot version which can be started
-from RAM. Therefore it mustn't (re-)configure the SDRAM controller.
-
-On 440EPx the SPL is copied to internal SRAM before the NAND controller
-is set up. While still running from cache, I experienced problems accessing
-the NAND controller.
-
-
-Example: Build and install NAND boot image for Sequoia (440EPx):
-
-a) Configure for sequoia with NAND boot support:
-# make sequoia_nand_config
-
-b) Build image(s)
-# make
-
-This will generate the SPL image in the "nand_spl" directory:
-nand_spl/u-boot-spl.bin
-Also another image is created spanning a whole NAND block (16kBytes):
-nand_spl/u-boot-spl-16k.bin
-The main NAND U-Boot image is generated in the toplevel directory:
-u-boot.bin
-A combined image of u-boot-spl-16k.bin and u-boot.bin is also created:
-u-boot-nand.bin
-
-This image should be programmed at offset 0 in the NAND flash:
-
-# tftp 100000 /tftpboot/sequoia/u-boot-nand.bin
-# nand erase 0 60000
-# nand write 100000 0 60000
-
-
-September 07 2006, Stefan Roese <sr@denx.de>