ARM: at91: remove partial parameter in bootargs for at91sam9x5ek.dtsi

Remove the malformed "mem=" bootargs parameter in at91sam9x5ek.dtsi

Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: stable <stable@vger.kernel.org> # 3.8+