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>