board/atmel/readme.txt: remove mention of sama5d4ek

The sama5d4ek defconfig has been removed in commit
9cb0b37cbd267ec1c2beea3deeaa86846bc2d037, but the board/atmel/readme.txt
file was not updated accordingly. This commit fixes that.

Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>