ARM: at91/sama5d34ek.dts: remove not needed compatibility string

"atmel,sama5ek" compatibility sting does not correspond to a
useful board configuration. This d34ek.dts is the only sama5d3
.dts file affected.

Reported-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>