boards/raspberrypi: don't document non-existent files

Those files were previously installed, but that's no longer the case
since we select in the menuconfig which version of the bootloader we
install.

[Thomas: fix minor typo in commit log.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/raspberrypi/readme.txt

@@ -55,9 +55,7 @@ After building, you should obtain this tree:
     +-- rpi-firmware
     |   +-- bootcode.bin
     |   +-- config.txt
-    |   +-- fixup_cd.dat
     |   +-- fixup.dat
-    |   +-- start_cd.elf
     |   `-- start.elf
     `-- zImage