ARM: at91/at91_dt_defconfig: remove memory specification to cmdline

No need for this cmdline option as we are using DT.
Moreover this defconfig is targeted to multiple SoC/boards: this option
was nonsense.
Reported-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/configs/at91_dt_defconfig

@@ -31,7 +31,7 @@ CONFIG_ZBOOT_ROM_TEXT=0x0
 CONFIG_ZBOOT_ROM_BSS=0x0
 CONFIG_ARM_APPENDED_DTB=y
 CONFIG_ARM_ATAG_DTB_COMPAT=y
-CONFIG_CMDLINE="mem=128M console=ttyS0,115200 initrd=0x21100000,25165824 root=/dev/ram0 rw"
+CONFIG_CMDLINE="console=ttyS0,115200 initrd=0x21100000,25165824 root=/dev/ram0 rw"
 CONFIG_KEXEC=y
 CONFIG_AUTO_ZRELADDR=y
 # CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS is not set