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>