treewide: mem: Enable MEMTEST via defconfig

There is no reason to enable MEMTEST from headers when was converted to
Kconfig already.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/ls1028ardb_tfa_SECURE_BOOT_defconfig b/configs/ls1028ardb_tfa_SECURE_BOOT_defconfig
index 4b61e78..6fa7f83 100644
--- a/configs/ls1028ardb_tfa_SECURE_BOOT_defconfig
+++ b/configs/ls1028ardb_tfa_SECURE_BOOT_defconfig
@@ -19,6 +19,7 @@
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 root=/dev/ram0 earlycon=uart8250,mmio,0x21c0500 ramdisk_size=0x2000000 default_hugepagesz=2m hugepagesz=2m hugepages=256 video=1920x1080-32@60 cma=256M"
 CONFIG_CMD_GREPENV=y
+CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_DM=y
 CONFIG_CMD_GPT=y
 CONFIG_CMD_I2C=y